IT’S BECAUSE OF ALLAH!!
“In The Name of ALLAH, the Most Merciful & Mighty” “BE FAITHFUL TO YOUR DUTIES ALWAYS AND EXPLORE THE HIDDEN TREASURES”
DEDICATION My whole work is dedicated to my Parents, Sisters, Teachers & My Friends and especially to my Sweet brother Amjid Noor Khattak whose valuable ideas and motivation made me able to touch this Stage and to my loving Uncle Habib Noor(Late).
PREFACE Like every person I have enjoyed the taste of my student life of 22 years with some strong observations and some experiments at extreme. Pertaining to my capacity I have put my effective efforts to consult different books and mega essays to augment and to put together my knowledge in constructive way, have cashed the golden opportunity of meeting with outstanding personalities who were expert in their respective fields and also I come across with their beneficial expertise which they were possessing in their respective fields. But at every moment of the life I just come across with an intense feeling to come up with some different and innovative product specially to throw light on our concealed treasure in our sweet homeland which is hidden and needs our due attention. It is an old saying that a success is hidden in the person itself only to require some little but revolutionary effort to reshape it and utilize it in such a way that it comes very true on this phrase that knowledge is that source of wealth which gets doubled while spending. In one of the donation meeting, designed for issuing reasonable amount as a loan for Pakistan to support of economy of Pakistan. But Japan was refused to make sure his contribution in granting donation for Pakistan by saying these historical statements.” Pakistanis are burning silver to bake their bread". Yes, a verbose statement is waiting for us to support that Pakistan is rich in mineral resources but only expecting from this youth to come up with zeal to explore this treasure and to reserve it, and to build this country’s economy in prosperous way and prove that our country needs no loan but only looking for peace and prosperity everywhere like this earth having enormous wealth but no atomic bomb, no cruse missile but leaving a message for all that our call our cry peace in the world, peace in the world. The purpose of this book is to supply the basic information about the study area Sanghar (Sindh) Pakistan. Using seismic Reflection Method to explore the natural Resources and to understand what lies beneath the surface of Erath. Using the data resulted from surveys conducted by Pakistan Oil and Gas Company Limited (OGDCL) the studied area is good reservoir area. This book contains little introduction to the Area, its General Geology, Geomorphology and Tectonic frame work. From exploration point of view this area is of very importance. Major structures observed in this area are discussed in very broad sense.
Successful completion of this book was not possible without the kind guidelines of my respected teachers Sir Dr.Zulfiqar,Mam Dr.Mona Lisa ,Sir Khalid Amin Khan and Sir Anees Ahmad Bangash.I acknowledge my work to my teacher Sir Dr.M.Idrees (GIK), Sir Imtiaz Ahmed (ICUP) and Sir M.Qasim (NUST) who have always been a source of inspiration and guidance for me all the way, whose valuable prayers, salutary advices and emboldening attitude kept my spirit alive to reach this milestone. The selfless, devoted and sincere cooperation of my senior Mr. Khurram Shehzad(Comsat), Mr.Manshad Rustam have been fortune to have a very nice company of friends Ubaid Zaman,Majeed-ur-Rehman,Amjid Qureshi,Fazle Ahad,M.Sabir,Zulfiqar Azeem,Ahmed Ali ,Lalzeb Khan and Imran Ramdani. Special credits and thanks to Mr.Shahid Nawaz who guided me in each and every step during the entire degree. I also acknowledge the help, the encouragement, endless love, support and prayers of my cousins Master Husnul Maab,Haider Zaman,Gohar Zaman,Akhtar Zaman,Abdul Basir and Akif Noor which have always been a source of inspiration and guidance for me all the way. MAJID KHAN KHATTAK M.Sc GEOPHYSICS April 2011.
CONTENTS S.No
Page# CHAPTER NO 1 INTRODUCTION TO AREA OF RESEARCH
1.1
Introduction
1
1.2
Base Map
4
1.3
Objectives
5
1.4
General Header Information
6
CHAPTER NO 2 GENERAL GEOLOGY AND TECTONICS OF THE AREA 2.1
Introduction
10
2.2
Regional Geological Settings
10
2.2.1
Northward Drift Of The Indian Plate And Opening Of The Indian Ocean
11
2.2.2
11
2.2.3
Formation Of Kohistan-Ladakh Island Arc And Its Collision With Eurasian Plate India-Eurasia Collision And Himalayan Upheaval
2.3
Tectonic Zones
12
2.4
Petroleum Geology Of Pakistan
14
2.4.1
Baluchistan Basin
14
2.4.2
Pashin Basin
14
2.4.3
Indus Basin
14
2.5
Geological Description Of Southern Indus Basin
16
2.6
History Of The Geological Evolution Of The Southern Indus Basin
17
2.7
Explanation Of The Structural Highs Of Southern Indus Basin
18
2.7.1
Thar platform
19
2.7.2
Karachi trough
19
2.7.3
Kirthar foredeep
19
2.7.4
Kirthar fold belt
20
2.7.5
Offshore indus
20
2.8
General Geology Of Sanghar Area
23
2.9
Tectonic Framework Of Sanghar Area
23
2.9.1
Sind Monocline
24
2.9.2
Tectonic Of Sind Monocline
24
2.10
Structural History
25
11
2.11
Generalized Stratigraphy Of The Sanghar Area
25
2.12
Major Formation Of The Area (Sanghar)
26
2.13
Hydrocarbon Potential Of The Area
32
2.13.1
Source Rocks
32
2.13.2
Reservoir Rocks
33
2.13.3
Cap Rocks
33
2.14
Future Prospects
35
CHAPTER NO 3 SEISMIC METHODS AND VELOCITIES 3.1
Introduction
36
3.2
Seismic reflection method
37
3.3
Seismic refraction method
39
3.4
Seismic Waves
41
3.5
Laws Governing The Propagation Of Seismic Waves
45
3.6
Seismic Velocities
47
3.7
Velocity Concepts
47
3.7.1
Rms (Root Mean Square) Velocity
47
3.7.2
Average velocity
48
3.7.3
Interval velocity
48
3.7.4
Stacking velocity
49
3.7.5
Migration velocity
49
3.8
Velocity Determination
49
3.9
Borehole Velocity Measurements Techniques
50
3.10
Velocity Variation
51
3.11
Compressional And Shear Velocities In Rocks
51
3.12
Uses Of Seismic Velocities
52
CHAPTER NO 4 SEISMIC DATA ACQUISITION AND PROCESSING
4.1
Introduction Seismic Data Acquisition
53
4.2
Energy sources
55
4.2.1
Impulsive energy sources
55
4.2.2
Non impulsive energy source
56
4.3
Acquisition setup
57
4.3.1
The spread configuration
57
4.3.2
Shooting types
59
4.3.3
Shooting parameters
59
4.3.4
Recording parameters
60
4.4
Detection and recording of seismic waves
60
4.5
Recording systems
62
4.5.1
Analog recording system
62
4.5.2
Digital recording system
63
4.6
Seismic noise
68
4.7
Noise Control
68
4.8
Introduction Seismic Data Processing
69
4.9
Processing in general
69
4.10
Processing sequence
70
4.11
Data reduction
70
4.12
Geometric Corrections
79
4.13
Data Analyzing And Parameter Optimization
82
4.14
Data Refinement
85
4.14.1
Stacking
85
4.14.2
Migration
86
CHAPTER NO 5 SEISMIC DATA INTERPRETATION 5.1
Introduction
75
5.1.1
Stratigraphical Analysis
76
5.1.2
Structural Analysis
76
5.2
Solving the Velocity Time Pairs
77
5.2.1
Root Mean Square Velocity and its Graph
77
5.2.2
Interval Velocity and its Graph
80
5.2.3
Average Velocity and its Graph
81
5.2.4
Mean Average Velocity Graph
83
5.3
Iso-Velocity Contour Map
85
5.4
Seismic Horizons
87
5.5
Seismic Time section
88
5.6
Seismic Depth Section
90
5.7
Contour Maps
92
5.7.1
Time Contour and Surface Map of Basal Sand Formation
92
5.7.2
Depth Contour and Surface Map of Basal Sand Formation
93
5.8
Reverse Modeling Of Seismic Section Into Impedance Section
95
5.8.1
Interval velocity
95
5.8.2
Instantaneous velocity
96
5.8.3
Root-mean-square velocity
96
5.8.4
Stacking velocity
96
5.9
Variations In Seismic Velocities
96
5.9.1
Lateral variations in seismic velocities
96
5.9.2
Vertical variations in seismic velocities
97
5.10
Correlation Between Velocity Types
97
5.10.1
The shear wave velocity
98
5.10.2
The density determination
98
5.10.3
The Compressional Wave Impedance
98
5.10.4
The Shear Wave Impedance
98
5.11
Compressional Wave Velocity Section Of Line 856-SGR-52
99
5.12
Shear Wave Velocity Section Of Line 856-SGR-52
100
5.13
Density Section Of Line 856-SGR-52
101
5.14
P- Wave Impedance Section Of Line 856-SGR-52
102
5.15
S -Wave Impedance Section Of Line 856-SGR-52
103
5.16
Compressional Wave Velocity Section Of Line 856-SGR-55
104
5.17
Shear Wave Velocity Section Of Line 856-SGR-55
105
5.18
Density Section Of Line 856-SGR-55
106
5.19
P -Wave Impedance Section Of Line 856-SGR-55
107
5.20
S -Wave Impedance Section Of Line 856-SGR-55
108
5.21
Well Correlations
109
5.22
Summary and Conclusions
110
CHAPTER NO. 1
INTRODUCTION TO THE AREA OF RESEARCH
NTRODUCTION TO THE AREA OF RESEARCH
1.1 INTRODUCTION The area of research is Sanghar, Khair Pur District (Sind province) belonging to Oil $ Gas Company (OGDCL) Pakistan. It is located in Lower Indus basin(Southern) bounded by Sargodha high in the north, Indian Shield in the east, Kirthar and Suleiman ranges in the west and Indus Offshore in the south. The basin is separated from Upper Indus Basin by Sargodha High and Pezu uplift in the north. The geographical coordinates of the area are: Latitude 27 43’ to 28 05’ North Longitude 69 35’ to 69 57’ East A seismic survey was carried out in Sanghar area by OGDC Pakistan in January-November 1985.The data acquisition and processing were made by selecting appropriate field and processing parameters. This dissertation pertains to the interpretation of 80 fold stacked and migrated time section of line number 856-SGR-52 and 856-SGR-55 oriented in NW-SE direction comprises of a total 556 and 1114 shot point’s form 101 to 667 and 101 to 1215. Normal faults (Horst and Graben Geometry) are more prominent structures of the area, which formed the structural trap. The identification of these traps is one of the main task for the oil exploration. To observe theses structures favorable for oil exploration the seismic lines 856-SGR-52 and 856-SGR-55 are provided by the department of Earth Sciences Quaid-iAzam University Islamabad, in order to interperate the seismic sections along the given seismic lines. The given lines belong to the Sanghar block which has many Oil and Gas discoveries. Survey was carried out by the OGDCL. Data provided is a shared data in which shot points were provided individually.
1
CHAPTER NO. 1
INTRODUCTION TO THE AREA OF RESEARCH
Figure 1.2 Location of Sanghar on Pakistan Map indicated by Star (www.mapzones.com).
2
CHAPTER NO. 1
INTRODUCTION TO THE AREA OF RESEARCH
Figure 1.3 A Satellite image showing Sanghar area (www.googlearth.com).
3
CHAPTER NO. 1
INTRODUCTION TO THE AREA OF RESEARCH
1.2 BASE MAP A base map typically includes locations of concession boundaries, wells, Seismic survey points and other cultural data such as buildings and roads, with a geographic reference such as latitude and longitude or Universal Transverse Mercator (UTM) grid information. Geologists use topographic maps as base maps for construction of surface geologic maps. Geophysicists typically use shot point maps, which show the orientations of seismic lines and the specific points at which seismic data were acquired, to display interpretations of seismic data. Base Map of interest is given below in Fig.1.4.
Figure1.4 Base Map of the study area showing positions & nature of the seismic lines. The Blue and green colored lines are the lines of interest titled with 856-SGR-55 and 856-SGR-52 respectively (K-tron Precision Matrix).
4
CHAPTER NO. 1
INTRODUCTION TO THE AREA OF RESEARCH
1.3 OBJECTIVES The main objectives of the dissertation are; To determine the times for each CDP at some constant interval on the basis of variation of these velocities in each velocity panel. To determine the average velocities at certain constant interval of time from each CDP data. To determine the mean of all average velocities determined for each constant time from different methods. To prepare the Mean Velocity Graph of mean average velocity vs. selected constant time. To analyze the structure using the Time Section and the Dix Iso-Velocity contour map. To calculate the depth of each interface using the provided average velocity information. To study the effects of the velocity variation along the profile and its distribution in the subsurface. To prepare the Time Contour Map and Depth Contour Map using K-tron X-Work and surfer and interpret the subsurface structure. To develop an understanding of tectonic, stratigraphic and structural framework of the area.
To prepare two way Time contour maps, Depth contour and Surface Maps using the lines 856SGR-52,846-SGR-45, 856-SGR-55 and 856-SGR-62 of Basal Formation which is a reservoir formation. For this purpose K-tron X-Work and surfer softwares are used.
Reverse Modeling of Seismic section into Impedance section using Advance surfer. Correlation of four wells by Log plot software using well tops data.
5
CHAPTER NO. 1
INTRODUCTION TO THE AREA OF RESEARCH
1.4 GENERAL HEADER INFPRMATION Information Reel number
856-SGR-52
856-SGR-55
T – 406
T-194
Number Sample /Traces
1500
1500
Sample Rate (In miles)
4
4
Line number
GR52
GR55
Job number
OGDC
OGDC
Section number
1
Processing step
STK
1 STK
SURVEY PARAMETERS (Field Information) The seismic lines 856-SGR-52 and 856-SGR-55 were recorded with the following parameters.
Information
856-SGR-52
856-SGR-55
Area
Sanghar(Sind)
Sanghar(Sind)
Recorded by
OGDC
OGDC
Field reels
027-049
411-462
Directions
W-E
S-N 55
Crew
SP-6
SP-6
Dates Recorded
Jan 10-14,1985
May24-Jun13,1985
52
SOURCE PARAMETERS Information
856-SGR-52
856-SGR-55
Energy source
Dynamite
Dynamite
6
CHAPTER NO. 1
Station interval
INTRODUCTION TO THE AREA OF RESEARCH
50m
50m
2m
2m
9
9
Depth No. of holes
RECEIVER PARAMETERS Information
856-SGR-52
856-SGR-55
Geophone type
MARK L-10 20HZ
MARK L-10 20HZ
Geophone code
03 12 05
03 12 05
Geophone interval
5m
5m
Group interval
50
50
Group length Spread Recorder Sample Rate
105m
105m 2550 – 200 – 0 – 200 – 2550 M SN338HR
258
2550 – 200 – 0 – 200 – 2550 M SN338HR
258
2 msec
2 msec
6 sec
6 sec
Tape Format
SEG – B
SEG – B
Tape Density
1600 BPI
1600 BPI
96
96
125 hz
125 hz
Record Length
Tape Channels Alias filter
7
CHAPTER NO. 1
INTRODUCTION TO THE AREA OF RESEARCH
PROCESSING SEQUENCE Demultiplex
Demultiplex to pettiy – ray mpx-1 format
Scale
Gain recovery curve applied
Balance
Scaling factor applied to data
Sort
Gather Traces Into Common Depth Order And Apply Field Statics
Filter
Band Pass Filter 4/8-50/60 Hz
Deconvolution
Time domain predictive Deconvolution applied using operator length= 256msec. Lagx=60msec. 0.5% white noise added prior to oper.calculation
Normal move out (NMO)
Normal Move out Corrections As Obtained From Analysis
Mute
Trace Suppression As Obtained From CDS
Stack
4800% CDP Stack
DISPLAY PARAMETERS Percent Gain Horizontal Scale
178 32.00 Tr/In
Vertical Scale
2.500 In/Sec
Polarity
Positive
Flimind Direction
RT0L
Percent Bar
-2989
Polarity
Black=Positive
Date Filmed
10/10/85 & 10/16/85.
Date Filmed
10/10/85 & 10/16/85.
8
traces
CHAPTER No.2
GEOLOGY AND TECTONICS OF THE AREA
ENERAL GEOLOGY AND TECTONICS OF THE AREA
2.1 INTRODUCTION The information about the geology of an area plays an important role for precise interpretation of seismic data, because some velocity effects can be generated from formation of different ideologies and also different velocity facts can be generated some lithological horizons. So as if we don’t know geological formations in area we don’t recognize the different reflections appearing in the seismic section.
2.2 REGIONAL GEOLOGICAL SETTINGS The Indian Ocean and the Himalayas, two of the most pronounced global features surrounded the Indo-Pakistan subcontinent, have a common origin. Both are the product of the geodynamic processes of sea-floor spreading, continental drift and collision tectonics. A plate of the earth’s crust carrying the IndoPakistan landmass rifted away from the super continent Gondwanaland followed by the extensive sea-floor spreading and the opening up of the Indian Ocean. Propelled by the geodynamic forces the Indian plate traveled 5000 Km northward and eventually collided with Eurasia. The subduction of the northern margin of the Indian plate finally closed the Neotythes and the Indian Ocean assumed its present widespread expanse. This collision formed the Himalayas and the adjacent mountain ranges. Pakistan has been divided into two broad geological zones, which are the; Gondwanaland Domain Tethyan Domain Pakistan is unique in as much as it is located at the junction of these two diverse domains. The southern part of Pakistan belongs to Gondwanian Domain and is sustained by the Indo-
9
CHAPTER No.2
GEOLOGY AND TECTONICS OF THE AREA
Pakistan Crustal Plate. The northern most and western region of Pakistan fall in Tethyan Domain and present a complicated geology. 2.2.1 NORTHWARD DRIFT OF THE INDIAN PLATE AND OPENING OF THE INDIAN OCEAN The Indo-Pakistan subcontinent separated from the Gondwana motherland about 130 million years ago (Jhonson et al. 1976). It has been estimated that between 130 m.y. and 80 m.y. India moved northward at a rate of 3 to 5 cm/year (Jhonson et al. 1976). From 80 m.y. ago India moved at an average rate of about 16 cm/year relative to Australia and Antarctica (Powell 1979). According to Patriat and Achache (1984), before anomaly 22 (50 m.y.) this rate of movement varied between 15 and 25 cm/year. 2.2.2 FORMATION OF KOHISTAN-LADAKH ISLAND ARC AND ITS COLLISION WITH EURASIAN PLATE The Neotethys had begun to shrink by the time Indian began its northward drift around 130 m.y. ago. Intra-oceanic subduction generated a series of volcanic arcs (Kohistan-Ladakh, Nuristan, and Kandahar) during the Cretaceous (Searle 1991, Treloar and Izatt 1993). This arc was intruded by 102 Ma pre-collision granitoids (Patterson et al. 1985, Treloar et al. 1989b, 1993) followed by the intra-arc rifting and magmatisim (Khan et al. 19930. Arc magmatism covered a life-span of about 40 Ma after which the back-arc basin closed and Kohistan-Ladakh arc collided with Eurasia along the southern margin of the Karakoram plate. 2.2.3 INDIA-EURASIA COLLISION AND HIMALAYAN UPHEAVAL The abrupt slowing down of Indian’s northward movement between 55 and 50 m.y. ago is attributed to this collision (Powell 1979). The abrupt slowing down of Indian from 18-19.5 cm/year to 4.5 cm/year occurred at 55+Ma. A combined India-Australia plate started moving away from Antarctica. Motion ceased along the former plate boundary (the Ninety East Ridge), and the Proto-Owen fracture no longer remained at transform fault, though it was reactivated later, about 20 Ma ago. Since 55 Ma ago India has steadily rotated counterclockwise. Coupled with Arabian’s separation from Africa about 20 Ma ago, this rotation caused convergence in Baluchistan, closure of some the some
10
CHAPTER No.2
GEOLOGY AND TECTONICS OF THE AREA
smaller basins (Scistan, Katawaz), collision various crustal blocks in Iran-Afghanistan region and formation of the Baluchistan fold-and-thrust belt. The India Eurasia collision produced the spectacular along uplifted and deformed 2,500 Km long Indo-Pakistan plate margin (kazmi & Jan, 1977).
2.3 TECTONIC ZONES Two broad geological divisions of this region the Gondwanian and the Tethyan domains are discussed. In this scenario Pakistan is unique inasmuch as it is located at the junction of these two diverse domains. The southeastern part of the Pakistan belongs to Gondwanian domain and is sustained by the Indo-Pakistan crustal plate. The northern most and western regions Pakistan fall in tethyan domain and present a complicated geology and complex crustal structure. On the basis of plate tectonic features, geological structure, orogenic history (age and nature of deformation, magmatism and metamorphism) and lithofacies, Pakistan may be divided into the following broad tectonic zones see fig2.1. (Kazmi A.H, et al. 1977). Indus Platform and fore deep. East Baluchistan fold-and-thrust belt. Northwest Himalayan fold-and-thrust belt. Kohistan-Ladakh magmatic arc. Karakoram block Kakar Khoarasan flysch basin and Makran accretionary zone. Chagai magmatic arc. Pakistan offshore.
11
CHAPTER No.2
GEOLOGY AND TECTONICS OF THE AREA
Figure 2.1 Map shows Tectonic Zones in Pakistan (Kazmi & Jan, 1997)
12
CHAPTER No.2
GEOLOGY AND TECTONICS OF THE AREA
2.4 PETROLEUM GEOLOGY OF PAKISTAN SEDIMENTARY BASINS Pakistan comprises following three sedimentary basins (Riaz Ahmed 1998). Baluchistan Basin Pishin Basin Indus Basin 2.4.1 BALUCHISTAN BASIN It is the second largest sedimentary basin of Pakistan measuring 149,000sq.km onshore where 8 well have so far been dressed without success. It is a Cenozoic subduction basin created as a result of oceanic slab belonging to the Arabian Plate under a block (Afghan) of the Eurasian Plate. Thick clastic sediments (>10,000m) deposited over arc trench gap from the sedimentary fill. (Riaz Ahmed, 1998). 2.4.2 PASHIN BASIN This basin is located between the Chaman Transform fault zone in the west and Muslim Bagh Waziristan ophiolite and melange zone in the south and east. It extends into Afghanistan where it is known as Katwaz Basin. The Pishin Basin is a small extra continental median basin of tertiary age (Ahmed, 1991) develop as a remnant of the Neo-Tethys ocean basin before the collision of the western margin of the Indian plate with the Afghan Block of Eurasian plate. The basin in terms of oil and bas exploration is considered as “frontier” because it totally unexplored. But its northeast segment will be the most attractive area for future exploration because of anticlinal structure. (Riaz Ahmed, 1998). 2.4.3. INDUS BASIN The Indus Basin belongs to the class of extra continental Trough Down warp basins. It is the largest and so for the only producing sedimentary basin of Pakistan. The basin is oriented in NE-SW direction. Basement is exposed at two places, one in NE (Sargodha High) and second in SE corner (Nagar Parker High). It is characterized by a large easterly platform region, which dips gently and monoclinally towards NW, a ring of Trough or depression in which platform dips and a westerly folded and thrusted topographically uplifted region. The convergence between Indian and Eurasian plate has resulted in
13
CHAPTER No.2
GEOLOGY AND TECTONICS OF THE AREA
partitioning of the basin into three parts. Upper, Middle and Lower called as northern, central and southern respectively. Some basement highs present over platform area serve as dividers. (Riaz Ahmed, 1998) Following is the classification of Indus Basin
Upper Indus Basin:
Kohat sub-Basin. Potwar sub-Basin.
Lower Indus Basin:
Central Indus Basin. Southern Indus Basin (Kadri, 1995).
LOWER INDUS BASIN BOUNDARIES OF LOWER INDUS BASIN Directions
Boundaries
East
Indian Shield
West
Kirthar and Sulaiman Ranges
North
Sargodha High
South
off Shore
The Lower Indus Basin is further divided into two classes, (a) Central Indus Basin (b) Southern Indus Basin The area provided to me (Sanghar) lies in the southern Indus basin. So I will mainly concentrate on southern Indus basin, its geology, tectonics and other aspects. (A)SOUTHERN INDUS BASIN The southern Indus basin (550 220 km) extends approximately between Lat. 24o and 28oN and from Long. 66o to eastern boundary of Pakistan (V. N. Qadri and S. M. Shoaib, 1986).It is characterized by several structural Highs. (a) Thar Platforms
14
CHAPTER No.2
GEOLOGY AND TECTONICS OF THE AREA
(b) Karachi Trough (c) Kirthar Foredeep (d) Kirthar Fold belt (e) Offshore Indus BOUNDARIES OF SOUTHERN INDUS BASIN
Directions
Boundaries
East
Indian Shield
West
Marginal Zone of The Indian Plate, Kirthar Range
North
Jacobabad KhairPur High, Mari Khandkot High
South
Offshore, Murray Ridge- Oven Fracture Plate Boundary
2.5 GEOLOGICAL DESCRIPTION OF SOUTHERN INDUS BASIN In the present plate tectonic setting, Pakistan lies between northwestern corner of the Indian plate, the southern part of the Afghan craton, and the northern part of the Arabian Oceanic plate. The eastern part of the Pakistan was affected by Tertiary plate convergence, having intense collision between the Indian and Eurasian continent in the Karakoram Thrust Zone to the north, and the translation between the Indo – Pakistan subcontinent ad the Afghan craton in the North West (Chaman Transform Fault). The western part of the country also affected by the Tertiary convergence between the Arabian Oceanic plate and the Afghan craton (Chagai Arc and the Makran Flysch Basin), and between a segment of Arabian Oceanic plate and the western rifted margin of the indo-Pakistan subcontinent. The western margin of the subcontinent (i.e. the eastern part of Pakistan) is characterized by a broad NS trending sedimentary basin (i.e. southern Indus basin) having thick Tertiary sequences underlain by Quaternary sediments. It had been relatively tectonically stable during the Mesozoic, but the intensity of shallow Tertiary folding increasing westward and becomes more pronounced in the strongly folded and faulted area of axial ford and thrust belt (N. A. Zaigham, 2000).
15
CHAPTER No.2
GEOLOGY AND TECTONICS OF THE AREA
2.6 HISTORY OF THE GEOLOGICAL EVOLUTION OF THE SOUTHERN INDUS BASIN Better understanding of geological evolution of the basin may provide strategies for new oil and gas discoveries in Pakistan. A geological history of the basin can be compiled by considering the basin forming tectonics and depositional sequence (Kingston et.al., 1983). The western margin of Indo-Pak continental plate is characterized by past extensional tectonics resulting in rifted protocontinent and new oceanic crust during sea floor spreading (Powell, 1979; Biswas, 1982; Zaigham 1991). The new oceanic crust was formed at a rate matching the continental separation similar to divergent associated with fossil rift in Africa, Europe and north Atlantic (Dingle & Scurtton, 1974; Jackson, 1980; Dickinson, 1982). Zaigham and Malik proposed a structural model for the evolution of southern Indus basin. 1. This corresponds to the initial rifting of the super continent Gondwanaland, probably during the Paleozoic (Smith & Hallam, 1970; Powell, 1979). The divergent phenomena includes the formation of Basaltic magma in the upper part of the Asthenoshpere, causing broad tectonic up warp and thinning of the overlying Lithosphere, probably resulting from plastic flow in the lower part and extensional faulting in the upper part. The thinning of Lithosphere continued and resulted in the collapse of the tectonic up warp over the magma blister and subsequently the process of sea floor spreading began with basaltic magma upwelling to the earth surface at oceanic Lithosphere. 2. Extensional forces broke the upper brittle crust into blocks separated by active faults during sea floor spreading. It appears that stretching of initial rifted part stopped at some geological time during very late Paleozoic to very early Mesozoic (Ahmed & Zaigham, 1993). The stretched crust remained as Indus basin failed rift in sediments started to accumulate. 3. The third step represents subsidence of the stretched continental crust and simultaneous accumulation of the Mesozoic and Tertiary sediments in the Indus basin. Thick Cenozoic strata are exposed along the western margin of the Indus basin in the Kirther Fold and Thrust Belt. A few small isolated outcrops of Tertiary are exposed near Khairpur in the northern part and Hyderabad in the southern part.
16
CHAPTER No.2
GEOLOGY AND TECTONICS OF THE AREA
Tertiary strata have also been reported from Jaisalmir and Ran Ketch areas (Biswas, 1982). Unconsolidated quaternary sediments rang between 30m and 200m thick in the southern Indus basin.
2.7 EXPLANATION OF THE STRUCTURAL HIGHS OF SOUTHERN INDUS BASIN The platform and trough extend into the Offshore Indus. The Southern Indus Basin is bounded by the Indian shield to the east and the marginal zone of Indian plate to the west. Its southward is confined by Offshore Murray Ridge-Oven Fracture plate boundary. The oldest rocks encountered in the area are of Triassic age. Central and southern Indus Basins were undivided until Lower-Middle Cretaceous when Khairpur-Jacobabad high became a prominent positive Feature. This is indicated by homogenous lithologies of Chiltan Limestone (Jurassic) and Sembar Formation (Lower Crataceous) across the High. Sand facies of Goru Formation (Lower-Middle Cretaceous) are also extending up to Kandhkot and Giandari area. This is further substantiated by Khairpur and Jhat Pat Wells located on the High. In Khair pur –2 well, significant amounts of Lower Cretaceous and Paleocene is missing while in Jhat Pat-01, the whole Cretaceous and Paleocene are absent with Eocene directly overlying Chiltan Limestone (Jurassic). Paleocene facies south of the High are quite different from those in North and are dominated by clasitic Sediments derived from the positive areas (Khairpur-Jacobabad High and Nabisar Arc). 2.7.1 THAR PLATFORM It is gently sloping Monocline analogous to Punjab Platform controlled by basement topography. The sedimentary wedge thins towards the Indian Shield whose surface expressions are present in the form of Nagar Pakar High. It differs from the Punjab Platform in that it depicts the buried structures formed due to extension tectonisim resulting from the latest counter-clockwise movement of the Indian Plate. It is bounded in the East by Indian Shield, merges into Kirthar and Karachi Trough in the West and is bounded in the north by Mari-Bugti Inner Folded Zone. A stratistructural cross section constructed through Thar Platform, Karachi Platform Trough and Offshore Indus. The Platform marks very good development of Early /Middle Cretaceous Sand (Goru) which are the reservoirs for all the oils/gas fields of Union Texas Pakistan and Oil Gas Development Corporation in this region.
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2.7.2 KARACHI TROUGH It is an embayment opening up into the Arabian Sea. The Trough is characterized by thick Early Cretaceous sediments and also marks the last stages of marine sedimentation. It contains a large number of narrow chains like anticlines, some of which contains gas fields (Sari, Hundi and Kothar). The Early, Middle and Late Cretaceous rocks are well preserved in this area. It has been a trough throughout the geological history. The Upper Cretaceous is marked by westward progradation of a marine delta. The most interesting feature of Karachi Trough is reportedly continued deposition across the Cretaceous/Tertiary (K/T) boundary wherein Korara Shales were deposited, the basal part of which represents the Danian sediments. This localized phenomenon probably represents a unique example where no Hiatus in sedimentation occurred at the end of Cretaceous era. Elsewhere, in Pakistan a break in deposition marked by laterites, Bauxites, Coal etc. is a common feature across the K/T boundary. 2.7.3 KIRTHAR FOREDEEP Kirthar Foredeep trends North-South which have received the sediments aggregating a thickness of over 15000 meters. It has a faulted Eastern Boundary with Thar platform. It is inferred that the sedimentation had been continuous in this depression. However, from the correlation of Mari, Khairpur and Mazarani wells it appears that the Upper Cretaceous would be missing in the area. Paleocene seems to be very developed in the depression but is missing from Khairpur-Jacobabad High area. This depression, like Suleiman Depression, is the area of great potential for the maturation of source rock. 2.7.4 KIRTHAR FOLD BELT This North-South trending tectonic feature is similar to Sulaman Fold that in structural style and stratigraphy equivalence. Rocks from Triassic to recent were deposited in this region. The configuration of the Kirthar Fold Belt also marks the closing of Oligocene- Miocene seas. The western part of Kirthar Fold Belt adjoining the Baluchistan Basin, which marks the western edge of the Indus Basin, is severely disturbed. This Western margin is associated with the hydrothermal activities which resulted in the formation of economic minerals deposits of Barite, Fluorite, Lead, Zinc, and Manganese.
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GEOLOGY AND TECTONICS OF THE AREA
2.7.5 OFFSHORE INDUS This area forms the part of passive continental margin and appears to have gone through two distinct phase of geological history (Cretaceous-Eocene and Oligocene-Recent). Sedimentation in the Offshore Indus started from Craterous time. However, deltaic and submarine fans sedimentation has occurred since Middle Oligocene time with the inception of Proto-Indus System. Offshore Indus is divided into platform and depression along a Hinge Line in close proximity and parallel to 67°East longitude. Offshore platform is divided into Karachi Trough and the Thar Platform’s deltaic area by a line which devudes Karachi trough from Thar Slope Onshore (Kadri, 1995).
Figure 2.2 Shows Sedimentary Basins of Pakistan (www.gsp.com.pk)
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GEOLOGY AND TECTONICS OF THE AREA
Figure 2.3 Shows the Stratigraphy of the Indus Basin, Each formation is shown by the specific symbol in the legend. (www.ogdcl.com).
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GEOLOGY AND TECTONICS OF THE AREA
2.8 GENERAL GEOLOGY OF SANGHAR AREA My Study area (Sanghar) is located in the Thar platform area of southern Indus basin. The Sanghar area is characterized by a series of horst and graben structure present almost below the base Paleocene unconformity within the cretaceous (Gilbert Killing et al, 2002). The southern Indus basin is identified as an extensional basin characterized by tectonic up warping on the western margin of the indo Pakistan subcontinent. Several hypotheses have so far been proposed to explain the origin of these crustal features, but these basement upwards remain puzzling (N. A. Zaigham) The project area lies between 26°, 19', 00" N to 26°, 24', 00" N (latitude) and 69°, 00', 00" E to 69°, 05', 30" E (longitude). Thar platform is gently sloping monocline analogous to Punjab platform controlled by the basement topography; the sedimentary wedge thins towards the Indian shield whose surface expressions are present in the form of Nagar Parker high. It differs from the Punjab platform in that it depicts the buried structures formed due to extension tectonism resulting from the latest counter clockwise movement of the Indian plate. It is bounded in the east by Indian shield merges into Kirthar and Karachi trough in the west and is bounded in the north by Mari-Bugti inner folded zone. The stratistructural cross-section constructed through Thar platform, Karachi trough and offshore Indus clearly shows the Stratigraphic and structural variation across the two sub basins. The platform marks very good development of Early/ Middle cretaceous sand Goru which are reservoirs for oil and gas (Kadri, 1995).
2.9 TECTONIC FRAMEWORK OF SANGHAR AREA The tectonic framework of the area of interest is defined by a collage of horsts and grabens that were originally formed during the Mesozoic rifting of India from Africa, Madagascar and Seychelles, and evolved during its subsequent drift to the north. Many of these structures are well defined on the Indian side, such as the Narmada, Kutch, and Cambay rifts and the Kathiawar horst, and are clearly visible on the satellite photographs. However, the situation is different across the border in Pakistan, where the rifted structures are masked by the Indus alluvium and collisional mobile belts of the Suleiman- Kirthar ranges and the Karachi Arc. It has been suggested (Wadia 1957, Biswas, 1982), that the Precambrian orogenic trends of peninsular India, such as the Dharwar (N-S to NNW-SSE), Narmada-Son (ENE-WSW), and Delhi-Aravalli
21
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GEOLOGY AND TECTONICS OF THE AREA
(NE-SW to E-W), were very influential in defining the Mesozoic rifting of India from the rest of the Gondwanaland. 2.9.1 SIND MONOCLINE The area of interest which is a part of Sanghar block lies in Sind Monocline is a part lower Indus basin situated on the Indo-Pakistan plate .It is triangular shaped with gentle slope towards. It is bounded in east by the Indian shield and merges into Kirthar and Karachi troughs in the west. In the north it is bounded by Sukkur rift zone. The Indus offshore platform in the south is the offshore extension of Sind monocline. Sind Monocline is an important oil and gas producing area of Pakistan where a large number of oil, gas and condensate fields have been discovered in the tilted fault traps formed as the result of extensional tectonics. In all the oil and gas fields, organic rich Sember shale deposited during Early cretaceous are the source rocks Late cretaceous upper Goru shale provided the excellent seal (Kadri, 1995). 2.9.2 TECTONIC OF SIND MONOCLINE Tectonics of Indian platform of which Sind monocline forms a part has been discussed by many authors. The northward movement of Indian platform generated compression where accompanying anticlockwise rotation produced tension. As a result of tension the platform was split into grabens and horst. This tectonic setting provided the ideal condition for widespread deposition of sediments exhibiting a variety of facies, including organic rich Sember shale (Source rock) and highly porous and permeable Lower Goru sands (Reservoir Rock) (Raza et al, 1990). Two sets of faults indicting two different episodes of rifting are developed in the platform. The first set of faults associated with early cretaceous Kutch rift phase and the second set is a consequences of Late cretaceous Cambay rift phase. A very investigation feature can be observed while looking at the map of these discoveries. The gas and condensate fields are concentrated in the north eastern and south western parts of monocline. Whereas the oilfields are restricted to the center of the area on the basis of the concentration of the gas, oil and condensate fields, the authors infer that late cretaceous Cambay rift divided Sind Monocline into Mithrao Tando Ghulam Ali Graben, Pakistan Bari Horst and Daru Nur Grabens.
22
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GEOLOGY AND TECTONICS OF THE AREA
High sand supply from an eastern south-eastern source, two prograding sand bodies, reaching sheet sand proportions, were deposited and form the main (Raza et al, 1990).
2.10 STRUCTURAL HISTORY Structural history of Sind area is characterized by extensional regime. Which produced normal faulting, and basement related structure within late Paleozoic to Quaternary sediments. These sediments were deposited on the peneplained Precambrian basements along the stable margin of Indian shield .A series of extensional events during the late Paleozoic time cretaceous as well as mid Tertiary collision between Indo-pak and Eurasian plate reactive old faults and produced new faults in the area. N-W oriented main structural features of Talhar fault zone are the results of this extensional tectonics (Kazmi, 1977).
2.11 GENERALIZED STRATIGRAPHY OF THE SANGHAR AREA Sanghar is located in the Thar platform of southern Indus basin. The Sanghar area is characterized by a series of horst and graben structure present almost below the base Paleocene unconformity within the cretaceous (Gilbert Killing et al, 2002). The extensional tectonism during the cretaceous time created the tilted fault blocks over a wide area of Eastern Lower Indus Sub BasinThe sedimentary section of Lower Indus Basin in South Eastern Pakistan consists mainly of Permian through Mesozoic sediments overlying a strong angular unconformity of presumably Late Paleozoic age. The whole area under investigation is overlain by alluvium as such no out crops are present at the surface which can yield a direct evidence of the stratigraphic succession. This part represents progradational Mesozoic sequences on a westward inclined gentle slope. Every prograding time unit represents lateral facies variations from continental /Shallow marine in the east to dominantly basinal to the west. In Thar slope area all Mesozoic sediments are regionally plunging to the west and are truncated unconformably by volcanic (Basalts of Khadro Formation) and sediments of Paleocene age. Permian Triassic and Lower Jurassic sediments in the Sindh area consists of inter-bedded sandstone, siltstone and a shale of continental to shallow marine origin. Platform carbonates (Chiltan Formation) conformably overly the lower Jurassic Clastics. Cretaceous sediments consist of a series of inter-bedded shale and sandstone and are divided into three separate formations termed as Sembar, Lower Goru and Upper Goru. The Sembar Formation which was not penetrated at any of the wells of this concession, directly overly the Jurassic carbonates and consists largely of shale with inter-bedded sands.
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CHAPTER No.2
GEOLOGY AND TECTONICS OF THE AREA
The lower Goru consists of sand sequences separated by shale and at places at marl units, the Upper Goru consists predominantly of marl and shale Tertiary sediments consists of inter-bedded sands and shale with massive Eocene carbonates (Gilbert Killing et al, 2002). A detailed discussion on the stratigraphic units of the study area is:
2.12 MAJOR FORMATION OF THE AREA (SANGHAR) Following are the major formations of Sanghar Area: 2.12.1 CHILTAN LIMESTONE The Chiltan limestone is typically a massive, thick bedded, dark limestone, but shows colour and texture variations within one section and in different areas. The colour varies from black, dark grey, grey, light grey, brownish grey, bluish grey to occasionally white. Pisolitic limestone beds are present locally. The texture varies from fine-grained, sub-lithographic to oolitic, reefoid and shelly. In the Axial Belt, the limestone gives a fetid smell. The Chiltan limestone is widely distributed in the Sulaiman Kirther Province and Axial belt, forming prominent high mountains like Koh-e-Maran, Koh-e-Siah, Chilian, Murdar Ghar, Takatu, Khalifat and Zardah. Most of these mountains are resistant cores of the anticlines. The limestone, where developed, overlies the Shirinab formation conformably. The upper contact with Mazar Drik formation is gradational. but in many areas this upper limestone (Mazar Drik formation) is not developed and recognized and the Chilian limestone has a disconformable contact directly with the overlying Sembar formation. The Chilian limestone correlates
with
the
Samana
Suk
Formation
of
the
Upper
Indus
Basin.
No fossils are found and on the basis of stratigraphic position age given is Middle Jurassic (Shah, 1977). 2.12.2 SEMBAR FORMATION The Sembar formation consists of black silty shale with interbeds of black siltstone and nodular rusty weathering argillaceous limestone beds. In the basal part pyritic and phosphatic nodules and sandy shales are developed locally. Rock unit is glauconite.And it is proven a good source rock. This rock unit is widely distributed in Sulaiman and Kirther ranges. Its Lower contact with various Jurassic formations such as Mazar Drik formation, Chilian limestone and Shirinab formation is disconfirmable while the upper contact is generally gradational with the Goru formation The Sembar formation is correlated with Chichali
24
CHAPTER No.2
GEOLOGY AND TECTONICS OF THE AREA
Formation of the Kohat-Potwar Province This rock unit is richly fossiliferous and the most common fossils reported are the belemnites, Mullucs and others and the age given is Early Cretaceous (Shah, 1977). 2.12.3 GURO FORMATION The Goru formation consists of interbedded sandstone, shale and siltstone. The limestone is grained, thin bedded, light to medium grey in color (Shah, 1977). On the basis of lithology Goru Formation is divided in two parts Lower Goru Upper Goru LOWER GURO The lower Goru is main reservoir rock within the area. The lower Goru horizon as a general 5 divisions based on predominant lithologies (Gilbert Killing et al).
The Basal Sand unit
Lower Shale
Middle sand unit (which has a good reservoir potential)
Upper Shale
Upper Sand
UPPER GURO The upper Goru sequence of middle to late cretaceous unformable overlies the lower Goru formation which consists of mainly marl and calcareous claystone occasionally with inerbeds of silt and limestone (Gilbert Killing et al). The Goru Formation is widely distributed in the Kirther and Sulaiman Province. The lower contact with the Sembar formation is conformable and is very locally reported unconformable by Williams (1959).The upper contact is transitional with the The Goru formation may be correlated with the Lumshiwal Formation of the Kohat-Potwar Province. The formation contains foraminifers and bivalves and age given is Early Cretaceous (Shah, 1977).
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CHAPTER No.2
GEOLOGY AND TECTONICS OF THE AREA
2.12.4 PARH LIMESTONE The Parh limestone is a lithologically very distinct unit. It is a hard, light grey, white, cream, olive green, thin-to-medium-bedded, lithographic and argillaceous limestone, with subordinate calcareous shale and marl intercalations.The formation is widely distributed in parts of the Axial Belt and Lower Indus Basin (Sulaiman and Kirther Province). The lower contact with the Goru formation is transitional and conformable, while the upper contact with the Mughal Kot formation is unconformable through most of its extent. The formation is correlated with the Kawagarh Formation of the Upper Indus Basin. The formation is richly fossiliferous. Forms (Globotruncana, Gumbelina) are dominant. No macrofossils are known. Age given is Late Cretaceous (S. M. Ibrahim Shah). 2.12.5 RANIKOT GROUP Blanford (1876) was the first to give the name Ranikot group.Vredenberg (1909a) subdivided the Ranikot group into Lower Ranikot (sandstone) an Upper Ranikot (limestone). One division of Ranikot group suggests that it comprise of three formations which are Khadro formation, consists of olive, yellowish brown sandstone and shale with interbeds of limestone. Keeping ascending stratigraphy order, Above Khadro formation is Bara formation (Lower Ranikot sandstone) consists of variegated sandstone and shale. and the upper one is the Lakhra formation (Upper Ranikot limestone) consists of grey limestone, grey to brown sandstone and shale. Various authers have given it different divisions. Below are explained the three formations as part of the Ranikot group with details (Shah, 1977). 2.12.5 KHADRO FORMATION The basal part of the formation is comprised of dark coloured limestone with shale, followed by olive, grey to green, soft, ferruginous, medium grained fossiliferous sandstone an olive, gery to brown gypsiferous shale with interbeds of fossiliferous limestone. A number of basaltic lava flows are also present. The volcanics contain dark green and black basalt interbedded with mudstone, claystone and sandstone ( Kazmi and Abbasi , 2008).
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GEOLOGY AND TECTONICS OF THE AREA
The formation is widely distributed in Kirther fold belt and its thickness varies at different localities. Its lower contact is unconformable with Moro formation and Pab Sandstone, while its upper contact is conformable with Bara and Dungan formations in various parts of Kirther-Sulaiman fold belt. Khadro formation may be correlated with the lower part of the Rakhshani Formation of Chagai and Ras Koh area. Fossils reported from the formation include Corbula Globigerina pseudobulloides and G. triloculinoides and so many others. And age given to the formation is Early Paleocene (Kazmi and Abbasi , 2008). 2.12.6 LAKI FORMATION Nuttal (1925) subdivided the laki formation into the following units.
Basal Laki Laterite
Meting Limestone
Meting Shale
Laki Limestone
The HSC (1960) collectively placed them under their “laki Group”. Cheema et al. (1977) proposed division of this formation into two units., namely sohnari member(Basal Laki Laterite ) at the base overlain by meting limestone and shale member, which include the three units of Nuttle, Meting Limestone, Meting Shale and Laki Limestone (Kazmi and Abbasi ,2008).
NARI GAJ SIWALIK AND ALLUVIUM FORMATION The remaining formation of Cenozoic era is considered as remnants. The formations have not any importance in term of oil and gas (Gilbert Killing et al, 2002).
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CHAPTER No.2
GEOLOGY AND TE TECTONICS CTONICS OF THE AREA
Figure 2.4 Shows the Stratigraph igraphic column of the Sanghar Area. Each formation is shown show with proper age. (www.ogdcl.com).
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GEOLOGY AND TECTONICS OF THE AREA
2.13 HYDROCARBON POTENTIAL OF THE AREA Oil production has been established in the Lower Goru sandstones, in Layers I, II and III of Cretaceous age. Progressive rifting of the Indo-Madagascan plate commenced, as stretch troughs, early in the Cretaceous period. During the initial phase of the evolution of the rift system the Sember formation with significant organic content was deposited under restricted circulation. The formation, along with the basal shales of the Lower Goru formation, represents the major source of hydrocarbon in the Lower Indus Basin. With the evolution of the rift system into a more mature half graben stage, the extensional tectonics resulted in tilted fault blocks over the Thar slopes. The lithosphere during the evolution of the rift system underwent readjustments causing subsidence and uplifts. Coupled with the worldwide eustatic pulses, the changes in sea level influenced the depositional environments, resulting in a sequence of delta-related sand bodies and marine shelf shale deposits. The tectono-eustatic oscillations also create a number of minor disconformities and marine transgressions. During mid-Cenomanian in one such marine submergence, “Badin shales” were deposited under more open marine environments and characterized by greater carbonate content (marls and thin limestone bands).After subsequent uplift, under very reservoirs in the region. Following a prominent depositional break, Turonian marine transgression created environments for pelagic sedimentation of the Upper Goru formation. These plastic marls and shales provide the capping mechanism with a thickness approaching 1,000 meters in the project area. Tilted fault blocks and horst draped by Upper Goru ductile lithologies, possibly, along the up dip truncation of Post-Badin shale sand bodies by a Turonian disconformities form the prevalent play types (ECL, 1988). 2.13.1 SOURCE ROCKS The lower Cretaceous Shale of Sember Formation are proven source for oil and gas discovered in the Lower Indus Basin because of its organic richness. Oil-prone kerogen type and thermal maturity. The Lower part of the Goru Formation is moderately rich in organic shale having fair to good genetic potential On lithostratigraphic correlation between Chak-5 dim Well No.01 and shahdadpur Well No.01 shale below massive sands are sufficiently well developed in the prospect area. This shale was geochemically analyzed at Shahdad pur well. Studies based on vitrinite reflectance infer that the kerogen is
29
CHAPTER No.2
GEOLOGY AND TECTONICS OF THE AREA
vitrinite dominated and immature up to a depth of 3345 m in Lower Goru Formation. Where amorphous organic matter rich oil prone kerogen become dominant and can be regarded as “Potential source rock” Pyrolysis analyses results of Shahdad pur Well indicate that in Lower Goru Formation from 3225 m to bottom depth, there is a zone of maturity transition from oil and wet gas generation to dry gas generation. 2.13.2 RESERVOIR ROCKS Basal Sands of Lower Goru (Cretaceous) Formation are the primary objective in this area. These sands are proven producer in Chack-01, Chack-05 dim south, Jakhro, Bobi and Kadanwari Fields. Average porosities for these sands are around 11% in the prospect area. 2.13.3 CAP ROCKS A thick stratigraphic sequence of shale and marl of Upper Goru Formation serve as cap rock for underlying Lower Goru Sand Reservoir. Shale of Lower Goru Formation also has the same properties.
Figure 2.5 showing petroleum zones of Indus Basin
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GEOLOGY AND TECTONICS OF THE AREA
2.14 FUTURE PROSPECTS 1. Aero magnetic data indicate a deep seated NS trending fossil failed rift in the Southern Indus Basin on the western margin of the Indo-Pakistan continental plate. 2. Horst and Graben structures have been identified in the subsurface associated with Indus fossil rift. 3. The deep and extensive sedimentary basin, known as the Southern Indus Basin, appears to have a developed owing to creation of the failed rift and subsequent subsidence and sedimentation processes. 4. The distribution of the seismic epicenters in the Indus plane exhibit a close association with the regional EW trending system of transcurrent faults related to the Indus Basin fossil rift, which seems to be active even at present, causing deformation of the overlying mountain ranges and geomorphological features. 5. The presence of the fossil failed rift has identified encouraging prospects of new discoveries of oil and gas in the Southern Indus Basin, because favorable conditions related to the sources, earth heat, reservoirs and seals prevail in the proposed geological models. The present basement interpretation, combined with the presence of known hydrocarbon source beds, reservoirs, and structures found in the surrounding basin should be further interpretated to delineate new exploration target as the vast tracts await drilling in the Southern Indus Basin.
31
CHAPTER NO.3
SEISMIC METHODES AND VELOCITIES
EISMIC METHODS AND VELOCITIES
3.1 INTRODUCTION Seismic Methods deal with the use of artificially generated elastic waves to locate hydrocarbon deposits, geothermal reservoirs, groundwater, archaeological sites, and to obtain geological information for engineering. It provides data that, when used in conjunction with other geophysical, borehole and geological data, and with concepts of physics and geology, can provide information about the structure and distribution of rock types. Exploration seismic methods involve measuring seismic waves traveling through the Earth. Explosives and other energy sources are used to generate the seismic waves, and arrays of seismometers or geophones are used to detect the resulting motion of the Earth. The data are usually recorded in digital form on magnetic tape so that computer processing can be used to enhance the signals with respect to the noise, extract the significant information, and display the data in such a form that a geological interpretation can be carried out readily (Kearey et al, 2002). The importance of the seismic methods over other geophysical methods as mentioned by Robinson & Coruth (1988) and Kearey & Brooks (1996) is due to its accuracy, resolution and presentation. In addition to oil and gas prospecting, the seismic methods are also employed for the: Measurement of the bedrock depth Ground water investigation Geotechnical purpose Followings are the main seismic methods:
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CHAPTER NO.3
SEISMIC METHODES AND VELOCITIES
3.2 SEISMIC REFLECTION METHOD The basic technique of seismic exploration consists of generating seismic waves and measuring the time required for the waves to travel from the source to a series of geophones, usually disposed along a straight line directed toward the source. From a knowledge of travel times to the various geophones, and the velocity of the waves, one attempts to reconstruct the paths of the seismic waves.
Figure 3.1 Seismic Reflection Geometry.
Figure 3.2 Basic layouts for Seismic Reflection Acquisition
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CHAPTER NO.3
SEISMIC METHODES AND VELOCITIES
Structural information is derived principally from paths that fall into two main categories: refracted types of path, the travel times depend on the physical properties of the rocks and the attitudes of the b paths in which the principal portion of the path is along the interface between two rock layers and hence is approximately horizontal; and reflected paths in which the wave travels downward initially and at some point is reflected back to the surface, the overall path being essentially vertical. For bothReflections of acoustic waves from the subsurface arrive at the geophones some measurable time after the source pulse. If we know the speed of sound in the earth and the geometry of the wave path, we can convert that seismic travel time to depth. By measuring the arrival time at successive surface locations we can produce a profile, or cross-section, of seismic travel times.
3.3 SEISMIC REFRACTION METHOD Refraction method is based on the study of elastic waves refracted along geological layer. This method is generally used for determining low velocity zone (weathered layer). There is one type of refraction, which gives rise to a phase that can travel back to the surface. This corresponds to the case of critical incidence. Seismic refraction method is helpful in the interpretation of seismic data (Al-Sadi, 1980). The waves which return from the top of interface are refracted waves, and for geophones at a distance from the shot point, always represent the first arrival of seismic energy (Telford, 2004).
Figure 3.3 Fundamental concept of refraction.
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CHAPTER NO.3
SEISMIC METHODES AND VELOCITIES
When an incidence wave crosses an interface between layers of two different velocities, the wave is refracted. That is, the angle of the wave leaving the interface will be altered from the incident angle, depending on the relative velocities. Going from a low-velocity layer to a high-velocity layer, a wave at a particular incident angle (the "critical angle") will be refracted along the upper surface of the lower layer. As it travels, the refracted wave spawns up going waves in the upper layer, which impinge on the surface geophones. Sound moves faster in the lower layer than the upper, so at some point, the wave refracted along that surface will overtake the direct wave.
Figure 3.4 Seismic Refraction Geometry. This refracted wave is then the first arrival at all subsequent geophones, at least until it is in turn overtaken by a deeper, faster refraction. The difference in travel time of this wave arrival between geophones depends on the velocity of the lower layer. If that layer is plane and level, the refraction arrivals form a straight line whose slope corresponds directly to that velocity. The point at which the refraction overtakes the direct arrival is known as the "crossover distance", and can be used to estimate the depth to the refracting surface. Seismic refraction is generally applicable only where the seismic velocities of layers increase with depth. Therefore, where higher velocity (e.g. clay) layers may overlie lower velocity (e.g. sand or gravel) layers, seismic refraction may yield incorrect results. In addition, since seismic refraction requires geophone arrays with lengths of approximately 4 to 5 times the depth to the density contrast
35
CHAPTER NO.3
SEISMIC METHODES AND VELOCITIES
of interest , seismic refraction is commonly limited to mapping layers only where they occur at depths less than 100 fee (Dobrin, 1988).
3.4 SEISMIC WAVES Seismic waves are messengers that convey information about the earth's interior. Basically these waves test the extent to which earth materials can be stretched or squeezed somewhat as we can squeeze a sponge. They cause the particles of materials to vibrate, which means that passing seismic waves temporarily deforms these particles can be described by its properties of elasticity. These physical properties can be used to distinguish different materials. They influence the speeds of seismic waves through those materials (Robinson & Coruh, 1988). There are mainly two types of Seismic Waves:
Body waves
Surface waves
BODY WAVES These are those waves which can travel though the earth interior and provide vital information about the structure of the earth. The body waves can be further divided into the following;
P- waves (Primary waves)
S- waves (Secondary waves)
P-WAVES (PRIMARY WAVES) The particular kinds of waves of most interest to seismologists are the compressional or P-waves also called as compressional waves, longitudinal waves, primary waves, pressure waves, and dilatation waves (see Fig. 3.5). In this case the vibrating particles move back and forth in the same direction as the direction of propagation of waves. P-waves can pass through any kind of material solid liquid or gas. The P-waves velocity depends upon density and elastic constants (Dobrin, 1976). The seismic velocity of a medium is a function of its elasticity and can be expressed in terms of its elastic constants. For a homogeneous, isotropic medium, the seismic P-wave velocity Vp is given by;
36
CHAPTER NO.3
SEISMIC METHODES AND VELOCITIES
Where: μ is the shear modulus. k is the bulk modulus. ρ is the density of the medium.
Figure 3.5 the propagation oof P-waves in an Elastic Medium.
S-WAVES WAVES (SECONDARY WAVES) In shear waves, the particles vibrate in a direction perpendicular to the direction of propagation of waves (see Figure 4.6). ).
Figure 3.6 the propagation of S-waves in an Elastic Medium.
37
CHAPTER NO.3
SEISMIC METHODES AND VELOCITIES
They are also called as Shear waves, transverse waves, and converted waves. For ideal gases and liquid μ=0. S-waves waves cannot pass through fluids. The velocity of S S-waves ves is given by (using the same notation as of Vp) (Dobrin, 1976).
CHARACTERISTICS OF BODY WAVES
These waves travel with low speed through layers close to the earth’s surface, as well in weathered layers (Robinson & Coruh,) .
Frequency
of
body
waves
in
exploration ation
vary
from
15Hz
to
100
Hz
(Parasnis, 1997). SURFACE WAVES A part from body waves more complicated patterns of vibration are observed as well. These kinds of vibrations can be measured only at locations close to the surface. Such vibrations must result from waves that follow paths close to the earth's surface, hen hence ce known as surface waves. In a bounded elastic solid, surface wave can propagate along the boundary of the solid. Frequency of surface waves is less than 15Hz (Parasnis, 1997) 1997). Surface waves are also of two types;
Raleigh waves
Love waves
RALEIGH WAVES Type of surface waves having a retrograde, elliptical motion at the free surface of a solid and it is always vertical plane. Raleigh waves are principal component of ground roll. The Figure 4.7 shows the propagation of Raleigh waves in an elastic medium (Kearey, 2002). 2002)
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Figure 3.7 Propagation of Raleigh waves in an Elastic Medium.(Kearey, 2002) LOVE WAVES A type of surface waves having a horizontal motion i.e. transverse to the direction of propagation. The velocity of these waves depends on the density and modulus of rigidity and not depends upon the bulk modulus (k). The Figure 4.8 shows the propagation of Love-waves in an elastic medium (kearey, 2002).
Figure 3.8 Propagation of Love waves in an Elastic Medium.
3.5 LAWS GOVERNING THE PROPAGATION OF SEISMIC WAVES There are the following laws usually governs the propagation of seismic waves.
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HUYGENS’S PRINCIPLE This principle states, “each point on a wave front may be considered as source of new wave front. FERMAT’S PRINCIPLE It is defined as “the elastic waves between two points along path requiring the least time”. SNELL’S LAW The direction of reflected and refracted waves traveling away from a boundary depends upon the direction of the incident waves and speed of the waves. Fig.4.9 a Sini / Sinr = VO / V1 Where, i
=
angle of incidence
r
=
angle of refraction
VO =
velocity of 1st medium
V1
velocity of 2nd medium
=
The angle of incidence “i” for which angle of refraction is 90o, is called “Critical Angle” and for refraction is denoted by ic Fig 4.9 Sin ic = Vo / V1
Figure 3.9 Showing Refraction and angle of incidence.
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3.6 SEISMIC VELOCITIES INTRODUCTION An accurate measurement of seismic P-wave velocities in various rock types is a crucial and an important step in seismic data interpretation. The accuracy of the data reduction, processing and interpretation of seismic data depends mainly on the correction of velocity measurements. The velocity of propagating wave depends upon the physical characteristics of the medium. In term of bulk modulus (incompressibility), modulus of rigidity µ, density ‘ρ’ the P-wave velocity is given by: Vp = [k + 1.333µ] 1/2
OBJECTIVE Principle objective of seismic velocities is to convert time section into depth section to have structural as well as lithological interpretation.
3.7 VELOCITY CONCEPTS In seismic prospecting, we deal with a medium, which is made up of a sequence of layers of different velocities, so it is necessary to specify the kind of velocities that are used. In Geophysics the word velocity refers to several different concepts. The most common concepts of velocity are briefly the following: 3.7.1 RMS (ROOT MEAN SQUARE) VELOCITY When the subsurface layers are horizontal having interval velocities as V1, V2,-------Vn and two way time to the respective interfaces as t1, t2, -----------tn ; then VRMS for an n layer model is defined as: n
n
VRMSn2 = ∑ (V12 t1 ) / ∑ t1 i=1
i=1
Root Mean Square velocity is always measured from the surface to a perpendicular interface. VRMS may be derived approximately from CDP shooting.
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3.7.2 AVERAGE VELOCITY Average Velocity Vav, can be simply obtained by dividing depth (hn) by its travel time (tn), where n = 1, 2, 3--------k. k
k
Vav = ∑ hn / ∑ tn n=1
n=1
Vav is also measured from the surface down to the reflecting surface. 3.7.3 INTERVAL VELOCITY It is the velocity within a chosen time interval and may be expressed as: VINTn = hn / tn – tn-1 Where h is the layer thickness and t is two way travel time. Dix’s Interval Velocity is obtained by the Dix Formula (Dix 1955) given by: VINTi+1 = ([Ti+1. VRMS2 i+1) – (Ti . VRMSi 2)] / (Ti+1 – T) 1/2 T denotes two ways travel time to horizontal interfaces and VRMS is the Root Mean Square Velocity. 3.7.4 STACKING VELOCITY It is the velocity obtained from the application of normal move out (NMO) correction to common depth point (CDP) gather. The travel time equation for homogeneous two layer model with flat horizontal interface is written in term of horizontal distance source and receiver (X), velocity (VNMO) zero offset two way time to the reflection (To). TX = (To2 + X2 / VNMO2)1/2 VNMO obtained by the equation: VNMO = (X2 . (Tx2 – To2))
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3.7.5 MIGRATION VELOCITY It is the velocity that is used to migrate seismic data. Vmig = VNMO. COS α The best migration velocities are the borehole average velocities. Some times stacking velocities are straight away used for migration.
3.8 VELOCITY DETERMINATION Velocity measurement can be direct, requiring the existing of a borehole, or indirect, based on time differences observed on the surface and estimate of reflector depth or layer thickness. Various methods are used for velocity determination. In seismic Prospecting two main approaches are available for the velocity measurements.
By use of exploration oil well, this is a direct method. A velocity function
is
computed from the continuous velocity survey.
By use of reflection travel time during processing, this is an indirect method.
3.9 BOREHOLE VELOCITY MEASUREMENTS TECHNIQUES Continuous Velocity Logging (CVL). Check Shot Survey or Well Shooting Scheme. Up-Hole Survey. A) CONTINUOUS VELOCITY LOGGING (CVL) This method involves the use of a seismic pulse generator with a detector, which is permanently attached to it. It is gradually lowered down in the well. Changes of velocity with depth are recorded as a continuous curve, which is commonly known as Sonic Log. B) CHECK SHOT SURVEY OR WELL SHOOTING SCHEME In this method, a detector is lowered down in a well to a known depth and a shot is fired on (or near) the ground surface. The average velocity down to each location is computed from the observed travel time (after being corrected for slant path) and the given depth.
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C) UP-HOLE SURVEY This technique is applied for finding the velocity variation with depth of near surface layer. A detector is kept on the surface near the well and small energy source are fired at known depths in the bear hole with in near surface layer. For each explosion, the average velocity is computed from the observed travel time (after being corrected for the slant path)
3.10 VELOCITY VARIATION Velocity variation is basically concern with the information of the velocities given in the upper part of the Seismic Section. In the velocity windows, the root mean square velocity (Vrms) and Interval velocities (Vint) w.r.t their respective time and depth are given.With the help of Vrms and Vint, we are able to calculate the average velocity (Vav) by using the Dix Average Velocity formula. With the help of velocity information, the velocity contour are drawn which shows the vertical as well as lateral variations in velocities. The vertical variation is mainly due to the formational changes, overburden pressure and age factor etc, however lateral variations in velocities may be due to folding and dipping of strata. The graph, which shows the velocity contours, is called Iso-velocity Section.
3.11 COMPRESSIONAL AND SHEAR VELOCITIES IN ROCKS Material and Sources
Compressional Velocity (m/sec)
Shear Velocity (m/sec)
Granite
5520-5580
2870-3040
Granodiorite
4780
3100
Diorite
5780
3060
Gabbro
6450
3420
Basalt
6400
3200
Sand Stone
1400-4300
2600
Lime Stone
1700-6060
2880-3030
Table shows compressional and shears velocities in rocks.
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3.12 USES OF SEISMIC VELOCITIES The seismic velocities may be used to establish the followings: True depth. Stacking of seismic data. Migration of seismic data. Possible lithology determination. Possible porosity estimates. Overpressure zone.
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EISMIC DATA ACQUISITION AND PROCESSING
4.1 INTRODUCTION SEISMIC DATA ACQUISITION Acquisition of seismic data, it is the very first step in the seismic exploration. It is the procedure through which seismic reflection data is acquired. With the help of modern electronics and computer industry, acquisition becomes very easy. Acquisition starts from shot and ends at recording the seismic events through various steps. Different energy sources are used to produce seismic waves and array of geophones are used to detect the resulting motion of earth. The data is recorded in digital as well as analogue form (Keary et al, 2002). Seismic surveys use low frequency acoustical energy generated by explosives or mechanical means. These waves travel downward, and as they cross the boundaries between rock layers, energy is reflected back to the surface and detected by sensors called geophones. The resulting data, combined with assumptions about the velocity of the waves through the rocks and the density of the rocks, are interpreted to generate maps of the formations (see fig. 4.1) After identifying sedimentary basins, prospects or fields thought to contain hydrocarbons, an oil company will then contract with a seismic acquisition company to map the areas underground rock formations through seismic surveying.
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Figure 4. 1Acquisition Process in work. Fundamental purpose of seismic data acquisition is to record the ground motion caused by a known source in a known location. First step in seismic data acquisition is to generate a seismic pulse with a suitable source. Second is to detect and record the seismic waves propagating through ground with a suitable receiver in digital or analogue form. Third is the registration of data on a tape recorder. Prior to seismic data acquisition, control points in the area under study are established using the satellite and local ellipsoid datum. These control points are used to control the exact orientation of seismic lines.
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4.2 ENERGY SOURCES The most common method applied in generating seismic waves is exploding dynamite in shot holes. There are, however, other methods which have been introduced as alternative seismic sources. Energy sources are categorized into two groups: Impulsive energy sources Non impulsive energy sources 4.2.1 IMPULSIVE ENERGY SOURCES The mechanism for generation of energy in this type is by exploding dynamite in a shot hole. Because of the impulsive nature of the seismic signal it creates and the convenient storage and mobility it provides for energy that can be converted into ground motion. An important example of impulsive energy source is dynamite (Yilmaz, 2001). DYNAMITE It is commonly used to generate sources, used in seismic prospecting. Generally it is exploded inside a drilled hole at a depth ranging from few meters to several tens of meters (See fig. 4.2). The deeper the charge the less intensive the generated surface waves are. It is advisable to place this charge below the weathering layer as it absorbs the high frequency components. Charge is usually sealed with water or mud to increase coupling with surrounding. Amounts of charge per shot point depend upon pattern of shooting (Telford, 2004).
Figure 4.2 Dynamite Shooting
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4.2.2 NON IMPULSIVE ENERGY SOURCE These sources involve mechanical impact upon the earth's surfaces or shaking of the surfaces with a mechanical vibrator. All sources of this type are so disposed in the field that signals received from impacts are applied to the earth over a linear distance comparable to what would be used for a line of shot holes at a shot point or over an area that would be used for two dimensional arrays of shot holes. (Robinson & Corch, 1988). VIBROSEIS It is based on the use of a mechanical vibrator which is hydraulically or electrically driven to exert a force, on ground surface, of an oscillating magnitude (see fig. 4.4). Its energy is called as sweep. (Al-Sadi, 1980)
Figure 4.3 Vibroseis truck in Action. Note the flat pad suspended under the middle of the truck. Once the truck reaches the specified point, this pad is used to raise the 20,000 kg truck entirely off the ground. The hydraulics then shakes the truck up and down on this central piston for a specified time and over a precisely controlled frequency band (e.g., eight 14 second sweeps from 10 to 56 Hz).
4.3 ACQUISITION SETUP The seismic acquisition setup involves the following The spread configuration
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Shooting types Shooting parameters
Recording parameters 4.3.1 THE SPREAD CONFIGURATION For acquisition of data and as well as to have quality of data high certain field operations are adopted. So the first step in this practice is the choice of spread type. The spread is defined as the lay out on the surface of, of the detectors which give recorded output for each source. Spread is made up of equal interreceiver distance and a defined offset. There is certain number of spreads called as basic spreads, which are used in seismic acquisition (see Figure 4.5). These spreads are:
End on spread
Inline offset spread
Split Spread/Centre shooting
Fan shooting
Figure 4.4 the basic spreads used in Seismic Acquisition. Along with these basic spreads, another technique called as Fan shooting can also be used. In this technique, geophones are arranged in an arc, fanning out in different directions from the source. Fan shooting is used in Transmission method. Transmission method differs from normal refraction method in the sense that it does not involve critical incidence of waves over the interface. In Transmission method, source and detector are on opposite sides of the investigated interface. Other techniques used in
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transmission method in velocity logging (well shooting, continuous velocity logging (CVL) and uphole survey.Fan shooting is used for determination of the dimensions of velocity anomalous structures (AlSadi, 1980). 4.3.2 SHOOTING TYPES There are different types of shootings used in the field. These types are:
Symmetric shooting (In this type the number of channels on sides of source is same.)
Asymmetric shooting (In this the number of channels on sides of source is not same)
End shooting (The source is at one end of the spread)
Roll along/Roll out shooting (In roll along method receivers are added in the spread while shooting in the source along the spread. Roll-out shooting is one in which the receivers are removed from the spread while shooting out along the spread.
4.3. 3 SHOOTING PARAMETERS Shooting parameters include:
Source size
Number of holes
Hole depth.
Shot at or between the pickets.
The shooting parameters are set by determining the following parameters:
Maximum offset ~depth deepest zone of interest
Minimum offset ~ not greater than shallowest section of interest
Maximum array length is determined by the minimum apparent velocity of reflections
Charge size determined by ambient noise late on the record
Line orientation (up-dip, down-dip)
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4.3.4 RECORDING PARAMETERS The recording parameters include: Group Interval Group Base Number of Channels Number of Geophones in a Group Geophone Array(Linear or Weighted) Sample Rate Record Length Coverage (Folds) Zero Offset and Common Offsets
4.4 DETECTION AND RECORDING OF SEISMIC WAVES After using energy sources, through which energy is supplied to earth, there comes the stage of detection and recording of seismic waves. In seismic detection it is necessary to detect vibration amplitude as small as 10-8 inches. Seismic equipment of adequate sensitivity, large dynamic range, and suitable frequency response is the aim of all development programs taking place in the field of seismic detection. Geophone is the most important detecting instrument used. (Al-Sadi, 1980) GEOPHONE The receiver used for the detection of ground vibration is called a geophone or a seismometer. It is used for seismic surveying on land, and it can also be operated on the ocean floor if mounted in a suitable container. Mechanism is that the motion of a coil around a magnet induces electric current to flow in the coil (see Figure 4.6). The strength of that current depends on the speed of the motion. The response of geophones to vibration of different frequency can be tested with a device called a shake table. The frequency of vibration that stimulates the strongest geophone response is recognized as the natural frequency of the geophone. It is found from the highest point on the response curve. Geophones commonly have natural periods in the range of 5 to 40 Hz. The smaller, more compact ones ordinarily
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have higher natural frequencies. The geophone is in a class of instruments that we call harmonic oscillators.(Robinson & Cotuch,1988)
Fig.4.5Basic Nomenclature of Geophone. SEISMIC CABLE Seismic cable is used to transmit geophone signal to recording system. Each geophone requires two wire conductors. Thus the number of conductors is double of geophones used in seismic cable. For small scale engineering surveys cable usually have 24 conductors which carry the signal of 12 geophones. The point where a geophone is connected to the pair of conductors is called the "takeout point". There are takeout points at regular intervals along the cable. (Robinson & coruch, 1988).
4.5 RECORDING SYSTEMS 4.5.1 ANALOG RECORDING SYSTEM For the first thirty years or so of seismic exploration, the outputs of the amplifiers were recorded directly on photographic paper by means of a camera. However, about 1952 recording on magnetic tape began. Today few seismic crews are not equipped for magnetic tape recording. The feature
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which originally led to widespread use of magnetic recording was the ability to record in the field with a minimum of filtering, automatic gain control, mixing etc., and then introduces the optimum amounts of these on play back. Analog magnetic tape recorders usually have heads for recording 26 to 50 channels in parallel (Telford et al. 1990). Analog systems are systems for which the input and output are analog signals i.e. continuous amplitude signal (Oppenheim, 2002). For an analog seismogram is a continuous record of ground motions as a function of time (fig. 4.6). The analog recording system is made up of an electric unit normally housed in recording station. Before the signal is recorded by analog system, it can be electronically amplified and filtered. The amplifier is used to increase the strength of weak geophone signals. Some of the signals may be removed by means of electronic filtering before recording the signals. An analog seismic recording system is equipped with a separate amplifier, filter circuit and a magnetic tape for each geophone. These components make up one channel of the recording system. 4.5.2 DIGITAL RECORDING SYSTEM One of the most significant developments in seismic technology has been introduction of the digital recording in the field first introduced into seismic work early in the 1960s. Digital recording represents the signal by a series of numbers which denote values of the output of the geophone (fig. 4.7) measured at regular interval, usually 2 or 4 milliseconds. A digital
Fig. 4.6 A Digital Recording System
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Fig. 4.7 A type of Roll along Switch: RLX 240 M (after Maurice and Sercel, 1997). Recorder makes use of binary numbers to store the measuring of geophone signal strength. In a multichannel system, each geophone signal is first amplified and filtered by analogue to digital converters (A/D Converter) and a procedure is accomplished by mean of a high speed switch called a Multiplexer. One of the most important advantages of digital seismic recording system is the large increase in dynamic range (i.e. 100 DB) over analogue system (Robinson & Coruh, 1988). The digital recorder makes use of binary numbers to store the measurement of geophone signal strength and has a significant advantage for the purpose of computer processing. The digital data recorded on a tape is in the form of binary numbers. Each digit of binary number on the tape is called “bit”. If the recording head magnetize this bit then it indicates “1” otherwise “0”.Digital recording system contains various units including Multiplexer, A/D converter, Formatter, amplifiers, filters etc. PRE-AMPLIFIER It is a fixed gain amplifier possibly a front end (fig. 4.9) of the seismic data acquisition (Maurice and Sercel, 1997), consisting of various number of preamplifier-filters equal to the number of seismic channels. The preamplifiers are followed by a multiplexer where the input signals are chopped into short time slices which are intermingled and queued through a single output channel. At this time, samples can be taken and held in a sample-and-hold amplifier. Each sample is raised to a proper voltage for accurate recording.
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Fig. 4.8 A complete recording unit with preamplifier unit labeled 1 and other parts. It shows how other units do are connected to pass their output to recorder and playback filters.
Fig 4.9 Basic Principle and procedure of a multiplexer. A multiplexer is a kind of selector switch connecting several inputs successively to a single output (A/D Converter, after Telford et al, 1990). It acts like the distributor of an internal combustion engine but in a reverse manner: the multiple inputs replace the connections to the spark plugs and the single output
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stands for the connection to the coil (fig. 4.10). The sliding contact D, turning at a constant speed, connects inputs 1, 2, 3, etc… one after the other and in this order to the output. After the last input, n, input number 1 is connected again to the output and the sequence repeats. For the processing all channels must be sorted out which is called Demultiplexing. AD-CONVERTER It is much easier to apply mathematical operations on digital data than on analog data. Therefore an analog signal has to be converted into a digital signal. Every Bit (0 or 1) corresponds to a certain voltage that each time differs with a factor two. The A/D-Converter compares the analog signal with the different voltage steps and add these in such a way that the smallest error between the analog and digital signal occurs. It is clear, that it is impossible to measure a signal which amplitude is larger than the sum of all separate steps or smaller than the smalls step. For a measurement, the gain must be set such that the amplitude of the measured data lies in the range of the AD converter. We speak of 8-bit, 16-bit, 20-bit und 24-bit sampling: Example: • 8-bit: 1 mV-256 mV • 24-bit: 1 µV-16 V SAMPLING By measurements using a digital system, the data is not continuously measured, but at a specific time interval measured and transported to the AD-converter. ALIASING It is a phenomenon during sampling usually occurs if higher frequencies are folded back into the Nyquist interval. Sampling frequency is the number of sampling points in unit time or unit distance. Thus if a waveform is sampled every two milliseconds (sampling interval: ∆t=0.002), the sampling frequency is 500 samples per second (or 500 Hz). Sampling at this rate will preserve all frequencies up to 250 Hz in the sampled function .This frequency of half the sampling frequency is known as the Nyquist frequency (fN) and the Nyquist interval is the frequency range from zero up to fN.
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fN
1 2 t
No information is lost as long as the frequency of sampling is at least twice as high as the highest frequency component in the sampled data. Seismic measurement systems have often an analog AntiAlias-Filter, which suppresses all Frequencies above the Nyquist-Frequency.
4.6 SEISMIC NOISE All type of disturbances created and interference with the signal of interest is called a noise. Noise is divided into two types. Coherent Noise. Incoherent Noise
COHERENT NOISE Coherent noise displays some regular patterns on a seismogram. Often it consists of recognizable waves such as surface waves, refracted waves and multiples that are produced by the source. INCOHERENT NOISE The coherent noises are caused by natural factors like rain, wind blowing, moving of vehicles etc. Incoherent noise displays no systematic pattern on seismogram.
4.7 NOISE CONTROL The basic tools available for controlling noise in the field include: Source size Source depth Electronic filtering Receiver arrays Electronic mixing
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4.8 INTRODUCTION SEISMIC DATA PROCESSING Data Processing is sequence of operations which is carried out according to a pre-define program to extract useful information from a set of raw data. It can be said “as an approach by which the raw data recorded in the field is enhanced to the extent that it can be used for the geological interpretation” (see figure 5.1). Data processing is to convert the information recorded in the field into a form that mostly facilitates geological interpretation (Al. Sadi , 1980). Seismic data processing strategies and results are strongly affected by field acquisition parameters. Additionally, surface conditions have a significant impact on the quality of data collected in the field. Lack of seismic reflected events on seismic section is not the result of a subsurface void of reflectors. Rather it is caused by low signal-to-noise ratio (S/N) resulting from energy scattering and absorption in the medium of propagation.
4.9 PROCESSING IN GENERAL Data Processing is a sequence of operation, which are carried out according to the predefined program to extract useful information from a set of raw data as an input-output system (Al. Sadi, 1980). Processing may be schematically shown as.
4.10 PROCESSING SEQUENCE The seismic data processing sequence can be broadly defined in five categories. Data Reduction Geometric Corrections Data Analysis and Parameter Optimization Data Refinement Data Presentation
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AIM AND PURPOSE The basic aim and purpose of data processing is to produce a perfect seismic section by applying a sequence of correction. Actually the seismic reflections from the depth are generally week and need to be strengthened by digital processing of field data (Robenson & Coruh, 1988). This approach involves the sequence of operation for improving signal to noise ratio(Dobrin & Sovit, 1988). The seismic field recorder generally records the data on magnetic tape. These tapes are then transferred to the data processing centre. Where the seismic data is processed. Processing seismic data consists of applying a sequence of computer program.
4.11 DATA REDUCTION Data reduction is done by certain processing operations as discussed below.
Demultiplexing
Geometry Definition
Correlation
Header Generation
Display
Editing and Muting
Amplitude Adjustment
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Fig.4.10 Detailed Processing Sequence Flow Chart. DEMULTIPLEXING Data recorded on digital magnetic tape is not suitable for analysis therefore it is assembled from the digital tape by a sorting process. Thus "the process of sorting data from the magnetic tape into individual channel sequence is called Demultiplexing”. Suppose there are four geophone arrays. Instantaneous voltage recorded by each geophone yields an array of samples. If each sample is identified by its geophone group source (A, B, C, D) and by its chronological sequence in that group (1, 2, 3, and 4).Then the output
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This scrambled sequence is called Multiplexed data, and the unscrambling multiplexed array into Trace Sequential Array is called Demultiplexing. The digital seismic data is recorded on magnetic tape by the recorder in the following way (Robinson & Coruh, 1988). After that data has been Demultiplexed, it is stored on tape in a convenient format in the following way, which is used in further processing.
GEOMETRY DEFINITION The layout of receivers for each shot record the location of all shots along the line, and all such field information must be described in detail to the computer for the geometry-specification step. Most geometry programs can access the digitized base-map file. Computer access is particularly necessary for processing crooked lines in which sources and receivers are not uniformly distributed along a straight traverse. The geometry program must calculate a source-receiver mid-point based on the two ground locations. All relevant geometric information is retained in the trace headers on the tape so that each trace is uniquely and accurately located. Later programs will time shift or filter as a function of ground location, offset, and/or other spatial coordinate(s) and time.
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CORRELATION Correlation is simply the measurement of similarity or time alignment of two traces. Since correlation is a convolution without reversing the moving array, a similar frequency domain operation also applies to correlation. (Yilmaz, 2001). There are two types of correlation; 1. Cross Correlation 2. Auto Correlation 1. CROSS CORRELATION Cross correlation measures how much two time series resemble each other. It is not commutative; output depends upon which array is fixed and which array is moved. As a measure of similarity, cross correlation is widely used at various stages of data processing (Yilmaz, 2001). For instance traces in a CMP gather are cross correlated with a pilot trace to compute residual static’s shift. It is the fundamental basis for computing velocity spectra. 2.
AUTO CORRELATION Cross correlation of a time series with itself is known as auto correlation. It is a symmetric
function. Therefore only one side of the auto correlation needs to be computed (Yilmaz, 2001). VIBROSEIS CORRELATION The signal generated by a Vibroseis is not a short pulse but rather a sweep lasting some seven to ten seconds. The sweep is transmitted through earth and reflected signal. Each reflection is a near duplicate of a sweep itself, so the reflections in Vibroseis record overlap act are indistinguishable. To make it useable reflections are compressed into wavelets through cross-correlation of data with original input sweep. After correlation each reflection on record looks similar to impulsive source data. This involves cross correlation of a sweep signal (input) with the recorded Vibroseis trace. The sweep is a frequency-modulated Vibroseis source signal input to the ground (Yilmaz, 2001). There are two types of sweep; 1. Up Sweep (When frequency of the Vibroseis source signal increases with time) 2. Down Sweep (When frequency of the Vibroseis source signal decreases with time)
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IMPORTANCE OF VIBROSEIS CORRELATION For Vibroseis source, we have a sweep (a train of waves) rather than a short pulse/source wavelet whereas most seismic impulsive sources generate a very short pulse which can be used directly to examine subsurface structure Vibroseis sweep lasts for several seconds depending upon the sweep time. So in case of Vibroseis source all reflected and refracted signals on a Vibroseis seismogram overlap one another extensively. Even after Demultiplexing of the Vibroseis seismogram it is impossible to recognize the reflections. So Vibroseis correlation procedure is applied.(Robinson & Coruch,1988). Vibroseis correlation enables us to extract from each of the long overlapping sweep signals on Vibroseis seismogram, a short wavelet much like those obtained with seismic impulsive source
EDITING AND MUTING Raw seismic data contains unwanted noise and sometime dead traces due to instrumental reasons. Thus the quality of data recorded is first observed by visual examination of raw field traces. Data may be affected by following reasons
Polarity reversals in data
Poor traces as well as poor bits
To remove polarity reversal, trace with reverse polarity is multiplied with it that becomes a trace with the polarity. Therefore editing is a process of removing or correcting traces, which in their original recorded taken, may cause stack deterioration (Rehman, 1989). After doing this all the contributing traces per each CDP are gathered together. Each trace in one CDP is identified by it s shot point and receiver numbers .The CDP-gathers may be displayed as such for direct inspection and checking of edited data. MUTING Trace- muting is a special type of data editing. This term is applied for process of zeroing the undesired part of a trace. In order to avoid stacking non-reflection events ( such as first arrivals and refraction arrivals) with reflection, the first part of the trace is normally muted before carrying out the stacking process .This is occasionally referred to as first break suppression (Al-Sadi,1980).
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Muting is useful to remove useless information from the processing stream in a way that first identifies the information to be removed and then blanked. Muting is categorized as Initial Muting, to remove first arrivals; usually done later in processing, and Surgical Muting, to remove air waves or ground roll energies.
AMPLITUDE ADJUSTMENT Amplitudes of the seismic wavelet is adjusted because it dies out as the input wave travels down to the earth and losses it energy due to the spatial spreading of the wave or absorption. Besides, spherical spreading and energy dissipation in earth, there are other reasons for the observable decay in seismic amplitude with time. Under the knowledge of such reasons amplitude of the seismic wavelet is adjusted: a. Trace Normalization b. Trace Balancing A. TRACE NORMALIZATION Trace Normalization is an amplitude adjustment applied to the entire trace. It is directly applicable to the case of a weak shot or a poor geophone plant. All absolute values of a trace are summed and compared with a reference value. A scaling factor is determined from the difference between the summation and the reference value, which is used to multiply all data with. Other possibilities of trace normalization could be Average value (Arithmetic or RMS), Median, Maximum Value to compensate the difference in amplitude which occurs due to the increasing distance between the source and receiver and the lateral differences in amplitudes. But the loss of amplitude with increasing depth is not taken into account. B. TRACE BALANCING-AGC The AGC function does not employ a gain to the whole trace, but employs a gain to a certain time sample within a time gate. First, the mean absolute value of trace amplitudes is computed within a specified time gate.Second, the ratio of the desired ‘RMS’ level to this mean value is assigned as the value of the gain function. This gain function is then applied to any desired time sample within the time gate; say the nth sample of the trace. The next step is to move the time gate one sample down the trace and compute the value of the gain function for the (n+1)th time sample and so on (fig. 5.2). The time gate
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is very important. Very small time gates can cause a significant loss of signal character by boosting zones that contain small amplitudes. In the other extreme, if a large time gate is selected, then the effectiveness of the AGC process is lessened. 256- to 1024-ms AGC time gates are commonly chosen. A disadvantage is that when the AGC gain is applied, it is not possible to reconstruct the original signal again. Therefore, the AGC is only used for display and printing purposes.
Fig. 4.11 AGC windows showing how it works. DISPLAY The data so processed is generally displayed in various modes (fig. 5.3) to summarize the information gathered. At any point of processing sequence the seismic analyst can display the data in wiggle trace or other modes. The choice of display is a matter of the client taste, but is not affected by company dictum. Currently, the data provided by OGDCL is the variable area with wiggles plot.
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Fig.4.12 Common Display types for Seismic Trace. AUTOMATIC GAIN CONTROL A grain recovery function is applied on the data to correct for the amplitude effects of wave front (spherical) divergence (Yilmaz, 2001). This amounts to applying a geometric spreading function, which depend upon travel time, and an average primary velocity function, which is associated with primary reflections in a particular survey area. Gain is applied to seismic data for spherical spreading correction. Often AGC (automatic gain control) is applied to raise the level of the weak signals. AGC attempts to make amplitudes similar for all off sets, for all time and for all mid points (Dobrin, 1988). A typical method of calculating the median or average amplitude with in sliding windows down the trace , then to calculate the multiples needed to equalize the median value in all the window. In interpretation of seismic section, variations in amplitudes of reflections can be the important factors .Lateral amplitude variations, from trace to trace, within a reflection event (bright spots) may be the direct indications of the presence of hydrocarbons. Vertical amplitude variations, from event to event, may be helpful in identifying and correlating reflecting horizons.
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4.12 GEOMETRIC CORRECTIONS In order to compensate for the geometric effects, we have to apply certain corrections on the recorded data .These corrections are called as geometric corrections (Dobrin, 1988). These corrections are applied on the traces gathered during trace editing and muting .The geometric corrections are 1. Static correction 2. Dynamic correction 1. STATIC CORRECTION Static correction compensates the effect of weathered layer and elevation effect due to unleveled surface .So static correction is of two types
Elevation correction
Weathering correction
For land data, elevation corrections are applied at the stage of development of field geometry to reduce the travel times to a common datum level (Yilmaz, 2001).This level may be flat or floating along the line. 2. DYNAMIC CORRECTION Dynamic correction compensates the effect of offset of receiver from the source .It is also related to the shape of the subsurface interfaces .It is also of two types.
Normal move out correction (NMO).
Dip move out correction. Normal move out correction is related more to the non-dipping interfaces. On the other
hand dip move out correction is related to the dipping reflectors. It accounts for the effect of dip of the subsurface interface along with the effect of offset distance of receivers (Robinson & Coruh, 1988). Dip-move out correction is applied to data following the normal-move out correction using flat-event velocities (Yilmaz, 2001). Figure 5.4 is the diagrammatical representation of the concept of static and dynamic corrections.
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Fig.4.13 Diagrammatic representation of static and dynamic corrections. TRACE GATHERING Traces are routinely gathered into groups having some common elements.
Common Source Point Gather.
Common Depth Point Gather.
Common Receiver Point Gather.
Common Offset Gather.
Common Mid Point Gather.
The concept of various types of Trace Gathers is shown in the Figure 5.5 as fallow:
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CMPGathers Fig.4.14 Diagrammatic representation of different trace gathers. The classical shooting pattern involves the procedure of a fixed shape spread, which moves along a linear profile at a regular move up rate. Such a spread is made up of equal inter-trace distances and a defined offset (Yilmaz, 2001). This technique ensures CDP coverage of a fold, which increases as the move up rate decreases. Multifold coverage can be calculated in terms of number of recording channels N, the geophone interval (X) and (S), and source interval as; Fold number = N X/2 S The ability to combine seismogram traces to obtain multifold reflection vastly improve signal to noise ratio. CDP technique is most common for data acquisition now days.
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4.13. DATA ANALYSING AND PARAMETER OPTIMIZATION There are three steps involved in the Data Analysis and Parameter optimization Filtering. Deconvolution. Velocity Analysis. FILTERING A filter is a system, which discriminates against some of its input. Seismic data always contain some signal information, which we want to preserve. Everything else is called noise, and we want to remove it. These systems, which are generally called filters work either by convolution in the time domain or by spectral shaping in the frequency domain. The common types of filters are the following: Low pass frequency filter. High pass frequency filter. Band Pass frequency filter. Notch filter. Inverse filter. Velocity filter. DECONVOLUTION It is the process by which the wavelet associated with the significant reflections is compressed and reverberatory energy that trails behind each reflection is largely attenuated .It is a filtering process designed to improve resolution and suppress multiple reflections. Deconvolution can be considered either in the time domain or in the frequency domain. In the time domain the object is to convert each wavelet with its reverberations and multiples, into a single spike. If we know the shape of the wavelet, we can design an operator which, when convolved with the seismic trace, with convert each wavelet into a single spike (Dobrin, 1988). It is a class of operations developed as a mean of partially reversing the effect of earth filter. When dynamite is blasted, spike is produced that is visible in the seismogram. Spike has very high frequency and short wavelength. When it travels through earth its amplitude decreases and it becomes a
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waveform, with lower frequency and greater wavelength. Thus earth is absorbing higher frequencies with time and depth. This behavior of earth is termed as hi-cut filter. Thus Deconvolution with a reverse process by which these higher frequencies are reproduced, called reverse filtering. Sometime- there are fake reflectors produced due to multiples which can cut by Deconvolution and deeper reflections become identifiable (Yilmaz,2001). VELOCITY ANALYSIS Velocity in seismic processing is an important parameter, which controls the stacking quality. Thus the proper velocity value gives the optimum dynamic correction which leads to efficient stacking process. The seismic traces of a common depth point gather are basis for each velocity analysis. Before velocity Analysis suitable static correction and data enhancement procedures are applied to the data (Yilmaz, 2001). A series of Normal Move Out corrections, each based on arbitrary constant velocity are then applied to each trace of data set. Then NMO corrected traces are stacked to produce a single output trace. This calculation is repeated for each constant velocity until the range of velocities applied extends from the minimum to maximum to be encountered in the area. The velocity increments may not be uniform but may be rather small for application of slower velocities, which yield large normal move out and large for higher velocities. A plot of velocities against record time for each analysis location represents the velocity function for that location. Velocity analysis is performed on selected CMP or CDP gathers. The out put from one type of velocity analysis is a table of numbers as a function of velocity vs. Two-way zero off set time also called as velocity spectrum. Numbers present in the table represent some measure of signal coherency along the hyperbolic trajectories governed by velocity, off set, and travel time. The curve in each spectrum represents the velocity function based on picked maximum coherency values associated with the primary reflections. The pairs of numbers along each curve denote the time_ velocity values for each pick. These velocity time pairs are picked from these spectra based on maximum coherency peaks to form velocity functions at analysis locations. In areas with complex structures, velocity spectra (defined above) often fail to provide sufficient accuracy in velocity picks. In that case, the data on staked with a range of constant velocities (called as
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constant velocity analysis), and the constant velocity stacks themselves are used in picking velocities (Yilmaz, 2001).
4.14. DATA REFINEMENT Processes described thus far are used to reformat, correct and diagnose data characteristics. In this step for data refinement the following procedures are carried out: STACKING MIGRATION 4.14.1 STACKING Once the necessary corrections have been applied, the data may be stacked. In the “corrected gather” the traces have been gathered into the depth order, both static and dynamics applied and the traces muted. All that remains to stack the data is to sum all the traces in each depth point, resulting in a single stacked traces being output for each depth point. Stacking is a data compression of one to two orders of magnitude. The signal-to-random noise ratio is increased through an N fold stack by N. after stack; the data are displayed at the surface location of the midpoint between source and receiver. When all adjustments to the data have transformed the offset data into time and phase coincidence with the zero offset traces, the common midpoint CMP and CDP are both widely often interchangeably. With dipping reflectors, the CMP after conventional processing is not the CDP. The correct positioning of reflection point will be by migration (Dobrin & Savit, 1988). Stacking chart is shown in Fig 5.6.
Fig. 4.15 shows Stacking Chart.
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4.14.2 MIGRATION Migration moves dipping reflectors into their true subsurface position and collapses diffractions, thereby delineating detailed subsurface features such as fault planes. So in this respect migration can be viewed as a form of spatial Deconvolution that increases the spatial resolution. Migration does not displace the horizontal events; rather, it moves dipping events in the up direction and collapses diffractions, thus enabling us to delineate faults.
Fig. 4.16 Seismic response form a dipping reflector, the recorded surface gives the apparent dip of the reflector surface. Therefore, migration is a tool used in seismic processing to get an accurate picture of the subsurface layer. It involves geometric repositioning of recorded signals to show a boundary or other structure, where it is being hit by the seismic wave rather than where it is picked up Now, not only the position but the dip angle can incorrectly imaged by vertically plotting (Rehman, 1989). TYPES OF MIGRATION With respect to the stage when migration is applied on the seismic data during processing, there are two important types of migration. Pre-Stack Migration. Post-Stack Migration.
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EISMIC DATA INTERPRETATION
5.1 INTRODUCTION Interpretation is the transformation of seismic data into structural and stratigraphic picture through a series of different steps. Thus threading together all the available geological and geophysical information including the seismic and then integrating them all in a single picture can only give a picture closer to the reality. The main purpose of seismic reflection survey is to reveal as clearly as possible, the structures and stratigraphy of the subsurface. The geological meanings of seismic reflection are simply indications of different boundaries where there is a change in acoustic impedance. These observed contrasts are associated with different geological structures are stratigraphic contacts. To distinguish different formations by means of seismic reflection is an important question in interpreting seismic reflection data. For this purpose we correlate the data with the well data and geology of the area under observation. The well data provides links between lithology and seismic reflections. The reflector identification is the next stage by which the actual interpretation starts and it establishes a stratigraphic frame block for the main interpretation. Extracting from seismic data the geological structures, such as folding and faulting are referred to as structural interpretation (Dobrin & Savit 1988). On the other hand, extracting non-structural information from seismic data is called, “Seismic Facies Analysis”. There are two main approaches for the interpretation of seismic section: Stratigraphic Analysis Structural Analysis
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STRATIGRAPHIC ANALYSIS Stratigraphic analysis involves the subdivision of seismic sections into sequences of
reflections that are interpreted as the seismic expression of genetically related sedimentary sequences. Basic principle in the seismic stratigraphic analysis is that reflections are taken to define chronostratigraphic units because interfaces that produce them are the stratal surfaces. Unconformities can be mapped from the divergence pattern of reflections on a seismic section. The presence of unconformable contacts on a seismic section provides important information about the depositional and erosional history of the area and on the environment existing during the time, when the movements took place. The success of seismic reflection method in finding stratigraphic traps varies with the type of trap involved. Most such entrapment features are reefs, unconformities, disconformities, Facies changes, pinch-outs and other erosional truncations. (Sheriff, 1999). Some of the parameters used in seismic stratigraphic interpretation are: Reflection Configuration Reflection Continuity Reflection Amplitude Reflection Frequency Interval Velocity External Form 5.1.2 STRUCTURAL ANALYSIS In structural interpretation main emphasis is on the structural traps in which tectonics play an important role. Tectonic setting usually governs which types of structures are present and how the structural features are correlated with each others, so tectonics of the area is helpful in determining the structural style of the area and to locate the traps. Structural traps include the faults, anticlines, duplex etc. (Sheriff, 1999). Seismic sections can predict the structure that scale up to few tens of kilometers. For large scale interpretation we have to use the grids of seismic lines. Unmigrated section is not suitable for structure interpretation, because it creates many problems like synclines becomes narrows and vice versa. Even a migrated section not fully fit for complex area like the area of study (Badley, 1984).
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Some seismic section contains images that can be interpreted without any difficulty. Discontinuous reflectors clearly indicate faults and undulating reflections reveals folded beds. Most interpretation of structural features are directly marked on seismic time sections (Robinson & Coruh 1988).
5.2 SOLVING THE VELOCITY TIME PAIRS The Root Mean Square Velocity functions given on a seismic section were processed by K-tron VAS (Velocity Analysis System) to compute interval & average velocity functions. The steps involved in the process are as follows; First we enter the velocity time pairs in notepad according to a particular format. Then using the sad software the R.M.S velocities can be converted into Interval velocities, and then from Interval to Average velocities. Then interpolate the velocity time (VT) pairs. At the end a mean Average velocity function can be generated using the Same software. The velocity information given on seismic section is in Root Mean Square Velocities (RMS) velocities. We have converted this RMS velocity into Interval and Average velocities, because for time to depth conversion of seismic section we need average velocity and interval velocity in order to find the rock and engineering properties. The R.M.S, Interval, Average and mean Average graphs are shown in the figures below. 5.2.1 ROOT MEAN SQUARE VELOCITY AND ITS GRAPH The weighted average velocity is called root-mean-square velocity (VRMS).When layers are horizontal with vertical velocities of VI,V2,V3,...,Vn and one-way time interval travel ti,t2,t3 ......... tn,, the root-mean- square velocity of the section of ground down to the nth interface is given by
V
RMS
N
N
V1 T0i 2
i
N
i1
T0i
1
2
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After putting the velocity function (RMS velocity) in VAS, RMS velocity graph is obtained. The variation of R.M.S velocity with time can be seen in Fig.5.1and Fig 5.2.This graph shows the lateral and vertical behavior of R.M.S velocity with time below particular CDPs.
Figure 5.1 Shows RMS velocity of the line 856-SGR-52
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Figure5.2 Shows RMS velocity of the line 856-SGR-55
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5.2.2 INTERVAL VELOCITY AND ITS GRAPH Interval Velocity (VN) is the average speed of wave front between two points measured perpendicular to the velocity layers.
V T V RMSN V N RMSN ON 2
2 1
T ON 1
T ON
1 2
The RMS velocity is then converted into interval velocity. Interval velocity graph can be seen in Fig. The interval velocities are helpful in process of migration and also in the estimation of Rock & Engineering properties.
Figure5.3 shows Interval Velocity of the line 856-SGR-52
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Figure5.5 shows Interval Velocity of the line 856-SGR-55 5.2.3 AVERAGE VELOCITY AND ITS GRAPH Average velocity (Vav) is simply the ratio of vertical depth to the travel time of a wave front from its source to that depth. Following Dix relationship is used to calculate the average velocity at specific interval of time for all the functions given in the section;
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Where Vav = average velocity (in m/s), Vint = interval velocity (in m/s) and T = zero of set travel time (in seconds). The interval velocity functions are further converted into average velocity functions that are significant in time to depth conversion of seismic section. Interpolated average velocity is used for depth conversion of the time section. The lateral and vertical variation represents the change in the subsurface lithology.
Figure5.6 shows the average velocity (interpolated) of the line 856-SGR-52
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Figure5.7 shows the average velocity (interpolated) of the line 856-SGR-55. 5.2.4 MEAN AVERAGE VELOCITY GRAPH Mean average velocity represents the mean of all the average velocities at any particular time. It shows the overall increasing trend of the average velocity with respect to time. It is used to calculate the mean average depth of seismic section.
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Figure5.8 shows mean average velocity Graph of the line 856-SGR-52.
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Figure5.9 shows mean average velocity Graph of the line 856-SGR-55
5.3 ISO-VELOCITY CONTOUR MAP Whenever the CDP’s plotted against the average velocities, a section formed which shows the lateral and vertical variations of the same velocity layers at different CDP’s. The map so formed is termed as the Iso velocity map. This graph shows the push-up and pull-up velocities. These velocities are representative of variation of average velocities at certain time along different vibrating points. To generate an Iso-velocity contour map K-tron VAS (Velocity Analysis System) generates a velocity grid by applying a temporal interpolation at 50 msec and spatial interpolation at 10 CDP intervals. The final velocity data is transferred to Golden Software Surfer using format translation programs written in OIL/Visual OIL (Khan et al., 2010) to generate the iso-velocity contour map.
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Figure5.10 Shows Iso-Velocity contour map of the Line 856-SGR-52
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Figure5.11 Shows Iso-Velocity contour map of the Line 856-SGR-55 Above Figures 5.10 &5.11 are the Iso-velocity contour maps of the seismic lines 856-SGR-52 and 856SGR-55 respectively. These maps shows the lateral and vertical variations in velocities along the seismic sections.
5.4 SEISMIC HORIZONS The main (Prominent) reflections that are present on the seismic sections are marked, and then selected those that showed good characteristics and continuity and can be traced well over the whole seismic section. There are difficulties in continuing the reflectors at the end of the seismic section and confusions are arrived where reflectors are mixed that may be due to sudden change in lithology, seismic noises, poor
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Data quality or presence of Salt in the subsurface at these locations. The seismic data was interpreted using K-tron X-Works application which provides interactive tools for marking horizons and faults. The seismic section was scanned as an image and loaded by the software X-Work a reference file is prepared which store the section in digital format. After referencing the section, horizons and faults are marked and the information that is the time at each point of the horizon and fault is stored in digital format and a geological cross section is prepared. Another advantage of the application is that the times for the prospective horizon can be sent to gridding and contouring software for generating time and depth contour maps. The software can also load the velocity functions and convert the time section into depth section. Since velocity varies vertically as well as laterally it does not apply a regional velocity function, instead it generates a velocity section which is used in time to depth conversion.
5.5 SEISMIC TIME SECTION After marking seismic horizons and faults, the time of each reflector was noted at different vibrating points, and then the seismic time section is generated by plotting the two-way travel time of the reflectors and faults on y-axis against the shot points on x-axis. The Seismic time section is simple reproduction of an interpreted seismic section. There are four reflectors on the time section with six faults on the Line 856-SGR-52 and five faults on the Line 856-SGR-55 with different orientation making Horst and Grabben geometries. The horst and grabben structure present on the seismic sections may be a suitable place for the accumulation of hydrocarbons. Time section is the developed section of reflectors, which shows subsurface structure in time domain. Time section of the Lines 856-SGR-52 and 856-SGR-55 are shown in Fig.5.12 and Fig.5.13 respectively. Reflectors are named on the basis of stratigraphic column using well top data.
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Figure 5.12 Seismic Time Section of Line 856-SGR-52(K-tron X-Work)
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Figure 5.13 Seismic Time Section of Line 856-SGR-55(K-tron X-Work)
5.6 SEISMIC DEPTH SECTION Vertical unit of seismic section represents two-way time, it does not show the true picture of sub-surface structure, so we convert the time section into depth section. Depth section is the conversion of seismic data from time to depth by using the relation (D= VT where D= depth, V = velocity & T is one wave time). Depth section of the given seismic line is generated by plotting the depth of the reflectors and faults on y-axis against the CDP #s (velocities are the functions of CDP #s) on x-axis as shown in Fig.5.14 & Fig.5.15 respectively.
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Figure 5.14 Seismic Depth Section of Line 856-SGR-52(K-tron X-Work)
Figure 5.15 Seismic Depth Section of Line 856-SGR-55(K-tron X-Work)
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5.7 CONTOUR MAPS Contouring is the main tool used in the seismic interpretation. After contouring it becomes obvious that what sort of structure is forming a particular horizon. The Basal Sand Formation was selected for the purpose of constructing contour maps because it is a producing reservoir in many areas. 5.7.1 TIME CONTOUR AND SURFACE MAP OF BASAL SAND The time contour maps have been generated using the Surfer (Software) surface mapping system version 8.0. For the time contour maps, the two way times of the interpreted reflector were recorded to create the XYZ data files. An XYZ data file is a file containing at least three columns of data values. The first two columns are the X and Y, latitude and longitude for the data points and third column is the Z value, which is the two-way time assigned to the shot point.
Fig 5.16 Shows Time Contour Map of Basal Sand (K-tron X-Work)
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Figure 5.16 Shows Time Surface Contour Map of Basal Sand 5.7.2 DEPTH CONTOUR AND SURFACE MAP OF BASAL SAND Velocities for each CDP of Basal Sand Formation are picked and multiplied the time with velocity to get the depth at each shot point.Then using the same procedure that is used in the preparation of the time contour maps. Below is given depth and surface contour map Basal Sand Formation. The surface map of depth is prepared to confirm the promising zone so the surface map also confirms the same structure.
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Figure 5.17 shows depth Contour Map of Basal Sand
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Figure 5.18 shows depth Surface of Basal Sand
5.8
REVERSE
MODELLING
OF
SEISMIC
SECTION
INTO
IMPEDANCE SECTION 5.8.1 INTERVAL VELOCITY If two reflectors at depths Z1 and Z2 give reflections having respective one-way times of t1 and t2, the interval velocity between Z1 and Z2 is defined as Vint = Z2-Z1/t2-t1
(Dobrin ,1976)
5.8.2 INSTANTANEOUS VELOCITY If the velocity varies continuously with depth, its value at a particular depth Z is obtained from interval velocity by contracting the interval ZI-Z2 to an infinitesimally thin layer having a thickness dZ. The interval velocity then becomes the derivative of Z with respect to “t”, which is the instantaneous velocity, defined as follows: Vinst = dZ/dt
(Dobrin, 1976)
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5.8.3 ROOT-MEAN-SQUARE VELOCITY When velocity layers are horizontal with respective interval velocities of V1,V2,V3,...,Vn, and oneway interval travel tunes t1,t2,t3 ......... tn, the root-mean- square velocity of the section of ground down to the n’th interface is given by Vrms=√ (sum of product of square of interval velocities and time/sum of one way travel times) The Rms velocity (Vrms) is always greater than the average velocity. The Rms velocity (Vrms) gives a better result, when single layer case is used for ∆t calculation. In fact, Rms velocity (Vrms) differs from the average velocity more and more as the layering becomes complex (Robinson & Coruh, 1988). Root-Mean-Square velocity gives better approximation of the travel time of a reflection in a multi-layer medium than the unweighted average velocity.(Al-Sadi, 1980). 5.8.4 STACKING VELOCITY It is the velocity obtained from normal move out measurements, used to maximize events in stacking process. It is approximately but not exactly same the Rms velocity. Stacking velocity is almost always greater than the average velocity
(Dobrin ,1976).
Stacking velocity Vst is based on the relation T2 = T2 +
X2 st Where, 0 V2 X = source-receiver offset for a CMP sequence of shots. T = travel time of the reflection at X. Tn = travel time at the zero offset/vertical travel time.
CONCLUSION Vav≤Vrms≤ Vst
(Dobrin, 1976)
5.9 VARIATIONS IN SEISMIC VELOCITIES There are two types of variations in seismic velocities. 5.9.1 LATERAL VARIATIONS IN SEISMIC VELOCITIES These variations are supposed because of slow changes in density and elastic properties due to changes in lithology or physical properties. Lateral variations make events appear to move up or down on time sections.
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5.9.2 VERTICAL VARIATIONS IN SEISMIC VELOCITIES These variations are due to lithological changes of layering and increasing pressure due to increasing depth. Normally seismic velocities increase with the increase in depth. (Robinson & Coruh, 1988) Vertical variation in velocity cause differences in the two way travel times of layers of equal thickness. (Lillie, 1999)
5.10 CORRELATION BETWEEN VELOCITY TYPES In seismic prospecting we are dealing with a medium which is made up of a sequence of layers of different velocities. In dealing with this kind of situation, it is necessary to specify the kind of velocity we are using. When velocity is measured for a defined depth interval, it is called as interval velocity and when it is determined for several layers it is called as average velocity. Relationship between interval velocity, Rms velocity and average velocity is given by “Dix Formula”. If we have Rms velocities (Vrms) then we can determine interval velocities (Vint) by using the following form of Dix formula. Sq. Vint,n =
(Sq.Vrms,n*Tn)-(SqVrms,n-1*Tn-1) Tn-Tn-1
Where Sq: Square function
(Al-Sadi, 1980)
If, on the other hand, we have average velocity (Vav), we can determine interval Velocity (Vint) as follows, using another form of Dix- formula. Vint,n =
(Vav,n*Tn)-(Vav,n-1*Tn-1)
Tn-Tn-1
(Al-Sadi, 1980)
Now, if we are given with interval velocities(Vint) and we have to determine average velocities (Vav), then Dix formula attains the form as given below Vav,n = (Vint,n*Tn-Tn-1)+(Vav,n-1*Tn-1) Tn
(Al-Sadi, 1980)
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So if we are given with any of the interval, Rms or average velocity, we can determine the remaining two by using the corresponding form of Dix-formula. 5.10.1 THE SHEAR WAVE VELOCITY The Shear Wave velocity is calculated by using the general equation given by Castagno in 1985, as Vp = 1.16Vs + 1360 For Vp & Vs are in m/sec. Shear wave velocity is calculated only as a parameter to compute the values of Modulii and Poisson’s Ratio as Modulii and Poisson’s ratio does not depend upon shear wave velocity but do depends upon compressional wave velocity. Also later on it was used in Amplitude Variation with Angle (AVA) analysis for Basal Sand Horizon. 5.10.2 THE DENSITY DETERMINATION Density is a major property of the rock which describes the amount of solid part of the rock body per unit volume. Simply mass per unit volume is called density. Higher denser rocks make the seismic velocity to drop down. The attenuation is higher for more dense rocks. The case is reverse for the lighter rocks. Seismic velocity is inversely proportional to density. Direct estimation of density from seismic velocities have been done by using the formula ρ = 0.31 * (Vp) 0.25 Where ρ = Density, Vp = P-Wave velocity in m/sec. Density is used in various reflectivity and modulii calculations.
5.10.3 THE COMPRESSIONAL WAVE IMPEDANCE The Compressional Wave Impedance is estimated by simply multiplying the Compressional Wave velocity with density. Mathematically, Impedance = ρ*Vp Where
ρ = Density, Vp = P-Wave velocity
5.10.4 THE SHEAR WAVE IMPEDANCE The Shear Wave Impedance is estimated by simply multiplying the shear Wave velocity with density. Mathematically,
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Where
ρ = Density, Vs = S-Wave velocity
5.11 COMPRESSIONAL WAVE VELOCITY SECTION OF LINE 856-SGR-52 The interval velocity variation along seismic line has been plotted with respect to their shot points and their respective time values. The purpose of this is to identify the interval velocity variations along the seismic line. There is not any abrupt drop or low velocity trap shown in the figure. The P-wave velocity is increasing uniformly with depth. This depicts different high velocity layers with increasing depth. The scale of the velocity section is shown in the figure.
Figure 5.19 showing the variation in Velocity along the Line 856-SGR-52
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5.12 SHEAR WAVE VELOCITY SECTION OF LINE 856-SGR-52 The S-waves Velocity variation along seismic line has been plotted with respect to their shot points and their respective time values. The purpose of this is to identify the S-waves velocity variations along the seismic line 856-SGR-52. There is not any abrupt drop or low velocity trap shown in the figure. The S wave Velocity is increasing uniformly with depth. This depicts different high velocity Layers.With increasing depth. The scale of the velocity section is shown in the figure.
Figure 5.20 shows different high velocity layers with depth.
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5.13 DENSITY SECTION OF LINE 856-SGR-52 The Density variation along seismic line has been plotted with respect to their shot points and their respective time values. The purpose of this is to identify the Density variations along the seismic line 856-SGR-52. There is not any abrupt drop or low Density trap shown in the figure. The Density is increasing uniformly with depth. This depicts different high density layers. With increasing depth. The scale of the density section is shown in the figure.
Figure 5.21 shows that density increases with depth
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5.14 P- WAVE IMPEDENCE SECTION OF LINE 856-SGR-52 The P wave Impedance variation along seismic line has been plotted with respect to their shot points and their respective time values. The purpose of this is to identify the Impedance variations along the seismic line 856-SGR-52. The P-wave impedance is increasing uniformly with depth. This depicts different high velocity layers with increasing depth. The scale of the velocity section is shown in the figure. There is one impedance trap shown in the figure. This trap is between the Times 3500 msec to 4000 msec between shot points 300-600. This can be a good hydrocarbon potential zone. This zone should be further studied with enhanced data and advanced approach.
Figure 5.22 shows P-waves impedance showing hydrocarbons Zone
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5.15 S -WAVE IMPEDENCE SECTION OF LINE 856-SGR-52 The S-wave Impedance variation along seismic line has been plotted with respect to their shot points and their respective time values. The purpose of this is to identify the Impedance variations along the seismic line 856-SGR-52. The S-wave impedance is increasing uniformly with depth. This depicts different high velocity layers with increasing depth. The scale of the velocity section is shown in the figure. There is one impedance trap shown in the figure. This trap is between the Times 3500msec to 4000msec between shot points 300-600. This can be a good hydrocarbon potential zone. This zone should be further studied with enhanced data and advanced approach.
Figure 5.23 shows S-waves impedance showing hydrocarbons Zone
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5.16 COMPRESSIONAL WAVE VELOCITY SECTION OF LINE 856-SGR-55 The interval velocity has been estimated for this seismic line of Sanghar survey. The interval velocity variation along seismic line has been plotted with respect to their shot points and their respective time values. The purpose of this is to identify the interval velocity variations along the seismic line. The velocity variation is very important as there is a significant seismic velocity drop in hydrocarbon potential area. There is not any abrupt drop or low velocity trap shown in the figure. The P-wave velocity is increasing uniformly with depth. This depicts the different high velocity layers with increasing depth. The scale of the velocity section is shown in the figure.
Figure 5.24 showing the variation in Velocity along the Line 856-SGR-55
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5.17 SHEAR WAVE VELOCITY SECTION OF LINE 856-SGR-55 The S wave velocity variation along seismic line has been plotted with respect to their shot points and their respective time values. The purpose of this is to identify the shear velocity variations along the seismic line 856-SGR-55. There is not any abrupt drop or low velocity trap shown in the figure. The S wave Velocity is increasing uniformly with depth. This depicts different high velocity layers with increasing depth. The scale of the velocity section is shown in the figure.
Figure 5.25 shows that with increasing depth velocity increases
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5.18 DENSITY SECTION OF LINE 856-SGR-55 The Density variation along seismic line has been plotted with respect to their shot points and their respective time values. The purpose of this is to identify the Density variations along the seismic line 856-SGR-55. There is not any abrupt drop or low Density trap shown in the figure. The Density is increasing uniformly with depth. This depicts different high density layers with increasing depth. The scale of the density section is shown in the figure.
Figure 5.26 shows that density increases with depth
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5.19 P -WAVE IMPEDENCE SECTION OF LINE 856-SGR-55 The P wave Impedance variation along seismic line has been plotted with respect to their shot points and their respective time values. The purpose of this is to identify the Impedance variations along the seismic line 856-SGR-55. The P-wave impedance is increasing uniformly with depth. This depicts different high. Velocity layers with increasing depth. The scale of the velocity section is shown in the Figure. There is one impedance trap shown in the figure. This trap is between the times 1700msec to 2100msec below shot points 300600. This can be a good. Hydrocarbon Potential Zone. This zone should be further studied with enhanced data and advanced approach.
Figure 5.27 shows P-waves impedance showing hydrocarbons Zone
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5.20 S -WAVE IMPEDANCE SECTION OF LINE 856-SGR-55 The S wave Impedance variation along seismic line has been plotted with respect to their shot points and their respective time values. The purpose of this is to identify the Impedance variations along the seismic line 856-SGR-55. The S-wave impedance is increasing uniformly with depth. This depicts different high velocity layers with increasing depth. The scale of the velocity section is shown in the figure. There is one impedance trap shown in the figure. This trap is between the Times 1700msec to 2100msec below shot points 300600. This can be a good hydrocarbon potential zone.
Figure 5.28 shows S-waves impedance showing hydrocarbons Zone
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5.21 WELL CORRELATION It is a direct method to investigate depositional setting of an area by correlating different stratigraphic successions encountered in different wells.
Figure 5.29 Stratigraphic correlations of four Wells showing difference in stratigraphical units within the area (Sanghar). Using well tops data of four wells Bobi-01, Barkat-01, Sachal-01 and Harthangu-01drilled at Sanghar area lithologies are studied by using Log Plot Software. To study the stratigraphic variations in an area usually wells are correlated. In well correlation we observe the thickness and depth of each strata. The figure shown is the output of the log Plot software 7 is the indication of the variations in stratigraphy of
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The study area that is the result of geological and tectonic settings of the area. Figure shows that its trend is towards east. Thickness of the alluvium decreases in trending direction and almost equal to zero in Harthangu well. Laki formation is exposed everywhere in the area.
5.21 SUMMARY AND CONCLUSIONS Based on the interpretation, following conclusions are made: On the basis of general stratigraphic column present in the area and the formation encountered in well BOBI #1, four reflectors are named. Reflector 1 is named as Kirther. Reflector 2 is named as Upper Goru Reflector 3 is named as Basal Sand. Reflector 4 is named as Chiltan. The seismic section shows a system of conjugate normal faults, making horst and graben structures. The Iso-velocity contour map shows vertical and lateral variation in velocity. There is a general trend of increase in the velocities with depth due to compaction and burial. Time to Depth conversion of seismic section gave a true picture of sub-surface structure. Time and Depth contour maps of Basal Sand helped to confirm the presence of horst and graben structures in the study area. Surface map of Basal sand gives the real shape of sub-surface structures, which are horst and graben. Horst structures are very suitable for trapping mechanism, to form a favorable prospect.
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Rock properties and engineering properties are calculated using well data; these are helpful to confirm the nature and type of sub-surface material. At the end using velocity data, a detailed analysis about Rock Physics of the area has been made by doing reverse modeling of seismic into impedance section of different rock physical properties for the location of any porous zone. After making a comprehensive analysis about Rock Physics, it is found that there exists some porous that is hydrocarbon potential zone. This zone should be further studied with enhanced data and advanced approach.
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