Basin to Basin: Plate Tectonics in Exploration

Ian Bryant Nora Herbst Houston, Texas, USA Paul Dailly Kosmos Energy Dallas, Texas John R. Dribus New Orleans, Louisiana, USA

The principles of plate tectonics help explorers understand and evaluate hydrocarbon plays. Since the start of the 21st century, these ideas have been successfully applied to presalt basins and turbidite fans along the coasts of South America and western Africa. Guided by global plate tectonics, exploration companies are applying winning play strategies from one coast of the South Atlantic to discover and prove similar plays on the opposite coast.

Roberto Fainstein Al-Khobar, Saudi Arabia Nick Harvey Neftex Abingdon, England Angus McCoss Tullow Oil plc London, England Bernard Montaron Beijing, People’s Republic of China David Quirk Maersk Oil Copenhagen, Denmark Paul Tapponnier Nanyang Technological University Singapore Oilfield Review Autumn 2012: 24, no. 3. Copyright © 2012 Schlumberger. For help in preparation of this article, thanks to Steve Brown, Copenhagen, Denmark; George Cazenove and Jonathan Leather, Tullow Oil plc, London; James W. Farnsworth, Cobalt International Energy, Inc., Houston; Winston Hey, Houston; Susan Lundgren, Gatwick, England; and Richard Martin and Mike Simmons, Neftex, Abingdon, England. Petrel is a mark of Schlumberger.

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New discoveries often emerge from previous successes. Once a play concept has proved commercially viable, oil companies are able to apply characteristics from their play to a regional or global framework in search of other accumulations. Through integration of exploration information, drilling data and geologic models from a successful play and through application of plate tectonic models, geoscientists are finding analog plays across ocean basins. From the North Sea to the Gulf of Mexico and from offshore South America to offshore Africa, explorationists have discovered major oil and gas fields in continental margin systems. The Santos, Campos and Espirito Santo basins off the coast of Brazil contain prolific oil discoveries, and the application of plate tectonic concepts has enabled explorers to extend that play across the Atlantic to offshore western Africa. Within the last few years, exploration companies have applied principles of plate tectonics to extend and relate upper Cretaceous turbidite fan plays westward— from West Africa across the Equatorial Atlantic to French Guiana and Brazil. This article describes some of the fundamental concepts that today’s geoscientists use to extrapolate plays across ocean basins. Case studies demonstrate how explorers have used plate tectonics and regional geology to expand exploration efforts in both directions across the Atlantic Ocean.

Basic Concepts Basins, petroleum systems and hydrocarbon plays are vital concepts in petroleum exploration. Basins collect the sediments that become the building blocks for petroleum systems. A petroleum system comprises an active source rock and the oil and gas derived from it that migrate to a reservoir and become confined there by a trap and seal.1 A play is a model used to explore for hydrocarbon deposits having similar characteristics. Petroleum systems may contain one or more plays, depending on the reservoir and style of trapping mechanism.2 Exploration experts systematically apply these concepts to locate prospects for drilling. Software platforms for databases, data integration and modeling are helping experts optimize their exploration workflows. A basin is a depression in the Earth’s surface that accumulates sediments. Basins form when the Earth’s lithosphere is stretched, fractured, loaded down or compressed in response to global tectonic processes. These processes also govern the size and depth—the accommodation space—of a basin, while climatic conditions determine water and sediment input for the basin fill material.

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Basins may be deformed by tectonic motion: extension, compression, strike-slip motion or any combination thereof. Extension may cause normal faulting and may be accompanied by stretching, thinning and sagging of the crust. Compression results in thrust faulting, folding, shortening and thickening. Strike-slip motion gives rise to translation and lateral faulting. A combination of these phenomena produces

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1. Al-Hajeri MM, Al Saeed M, Derks J, Fuch T, Hantschel T, Kauerauf A, Neumaier M, Schenk O, Swientek O, Tessen N, Welte D, Wygrala B, Kornpihl D and Peters K: “Basin and Petroleum System Modeling,” Oilfield Review 21, no. 2 (Summer 2009): 14–29. Stewart L: “The Search for Oil and Gas,” Oilfield Review 23, no. 2 (Summer 2011): 59–60.

2. Doust H: “Placing Petroleum Systems and Plays in Their Basin History Context: A Means to Assist in the Identification [of] New Opportunities,” First Break 21, no. 9 (September 2003): 73–83. Doust H: “The Exploration Play: What Do We Mean By It?,” AAPG Bulletin 94, no. 11 (November 2010): 1657–1672.

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O Overburden C Caprock R Reservoirs Source rocks

Tertiary

O C R

Clayey-sandy sediments

C

Marls

Oceanic crust

Continental crust

Cretaceous

C R Limestone C C Lithosphere

R

Salt Synrift lacustrine sediments

> Petroleum systems. Explorationists define the petroleum system as the geologic elements and processes that are essential for the existence of a petroleum accumulation. This cross section summarizes petroleum systems along a South Atlantic continental margin. The geologic elements must be present in the following order: The source rock contains organic matter, reservoir rock receives the hydrocarbons and has sufficient porosity and permeability for storage and recovery of hydrocarbons, sealing caprock is impermeable to keep the fluids in the reservoir and overburden rock buries the source rock to depths having the optimal temperature and pressure for source rock maturation and hydrocarbon generation. Rifting of the South Atlantic Ocean started with extension and faulting (black solid going to dashed lines) of continental crust (brown). The continental crust thinned and eventually split apart. As the two parts of the continental crust separated (only the right side is shown here), oceanic crust (gray) formed at a midocean ridge (not shown) during seafloor spreading. The continental margin is located where the thinned continental crust meets oceanic crust. Synrift lacustrine basins were preserved and filled with source (blue) and reservoir (white) rock that were eventually trapped and sealed underneath salt (purple). Hydrocarbons from synrift source rock migrated to limestone reservoirs (green bricks) that were buried and trapped beneath postsalt marls (green). The marls also provided source rock (dark green). During the Tertiary, clayey-sandy sediments (yellow and tan) buried the margin, providing source rock, reservoirs, caprock and overburden. [Illustration adapted from Huc AY: “Petroleum in the South Altantic,” Oil & Gas Science and Technology—Revue de l’Institut Français du Pétrole 59, no. 3 (May–June 2004): 243–253.]

pull-apart basins, push-up blocks and transtension or transpression oblique slip. Thus, local or large-scale movements provide the impetus for creation of stratigraphic or structural traps. Stratigraphic traps result from facies changes or juxtaposition of impermeable and permeable strata. Structural traps form as a result of strata deformation. The tectonic and stratigraphic history of a basin gives it a global and regional setting for its formation, filling and deformation.3

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Exploration teams composed of geologists, geochemists, paleontologists, geophysicists and petrophysicists unravel the history of a basin and sequence of tectonic events and cycles of sedimentation filling a basin. They identify source rocks within the basin and correlate them with known trapped hydrocarbons.The teams examine the geologic elements and processes that created known source rocks and traps to develop leads to other similarly generated accumulations (above). After further investigation, if the lead still appears to

have potential to trap hydrocarbons, it becomes a prospect.4 Once identified, the prospects are ranked according to uncertainty, risk, potential reward and market value of hydrocarbons. Integrated software systems that incorporate mapping and petroleum systems and play analysis tools, such as the Petrel E&P platform, help geoscientists evaluate basins (next page).5 Geoscientists use them to construct and share geologic models in 3D and provide an environment for storing data and models.

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Project and portfolio economics

Model-based interpretation from basin to prospect

Play and prospect evaluation

Geomechanics and seal analysis

Trap

Reservoir

Charge and timing petroleum system modeling

Structural restoration

> Exploration software platform. Exploration experts combine seismic information, well logs, geochemical and heat flow data and other geologic data to work from basin to prospect scale (clockwise top center to middle right). Regional to prospect scale models of traps (top right) and reservoirs (middle right) built in the Petrel platform benefit from integration with structural restoration tools (bottom right) and petroleum system modeling (bottom center). Both petroleum system modeling and structural restoration tools may be used to gain an understanding of the geomechanics of the basin to guide evaluation of seals (bottom left) and plan exploration wells. Risk assessment tools allow exploration teams to assign uncertainty and risk to acreage and drillable prospects (middle left). Petroleum economic evaluation enables planning exploration portfolios (top left).

By creating models at various scales, geoscientists are able to develop geocellular models from global to regional and local scales. This integration allows geoscientists to determine, for example, whether a particular local channel-levee interpretation is consistent with the regional interpretation or whether a widespread organic-rich facies mapped at the tectonic plate scale corresponds to source rock facies in the prospect model of the targeted petroleum system.

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3. A facies is a rock unit defined by characteristics that distinguish it from neighboring units. For more on stratigraphic and structural traps: Caldwell J, Chowdhury A, van Bemmel P, Engelmark F, Sonneland L and Neidell NS: “Exploring for Stratigraphic Traps,” Oilfield Review 9, no. 4 (Winter 1997): 48–61. For sequence stratigraphy: Neal J, Risch D and Vail P: “Sequence Stratigraphy—A Global Theory for Local Success,” Oilfield Review 5, no. 1 (January 1993): 51–62.

4. This chain of events from hydrocarbon source to its resting place in a distant reservoir applies to conventional petroleum systems. For unconventional systems, the source rock may also be the reservoir rock. Such unconventional systems include oil and gas from shale or coalbed methane. McCarthy K, Rojas K, Niemann M, Palmowski D, Peters K and Stankiewicz A: “Basic Petroleum Geochemistry for Source Rock Evaluation,” Oilfield Review 23, no. 2 (Summer 2011): 32–43. 5. Al-Hajeri et al, reference 1.

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Present day Jubilee discovery, Tano basin

Cretaceous

Zaedyus discovery, Guyana-Suriname basin Precambrian

Play projection Present day

Cretaceous

Precambrian

Tupi discovery, Santos-Campos basin

Extrusive volcanics Nondeposition Organic-rich clastics Lacustrine facies Deep marine sand-dominated clastics Paralic facies Deep marine carbonates Shallow marine carbonates Deep marine clastics Shallow marine clastics Terrestrial sediments

Azul and Cameia discoveries, Kwanza basin

> South Atlantic conjugate margins through geologic time. Two regional geologic models, built on opposing coasts of the South Atlantic, are constrained by a global sequence stratigraphic model. By assimilating interpretations into a 3D environment using the Petrel platform, geoscientists have derived a workflow to populate a tectonic plate–scale geocellular model for the sedimentary evolution of the margins through geologic time as illustrated in the exploded view of the South Atlantic continental margins from Precambrian time at the deepest surface to the present at the upper surface. Data assembled in this way on a common software platform allow explorationists to project petroleum system facies to a data-poor region by using sequence stratigraphy and elements of petroleum system modeling from a data-rich region to correlate and extrapolate associated facies. A recent example of this approach may be found along the transform margin where successful exploration concepts developed in Turonian-age lowstand turbidite fans offshore Ghana have been applied offshore French Guiana, leading to the recent Zaedyus discovery within similar deposits. Visualized in geologic time, these lowstand systems may be explored with their associated petroleum elements. Compelling evidence from wireline log responses, hinterland cooling events and biostratigraphically constrained unconformities were integrated; the results suggest that Campanian-age lowstand deposits may also provide attractive reservoir targets in the Guyana-Suriname basin offshore northern South America. The Campanian stratigraphic interval, while not as well tested as the Turonian interval, has also been attracting interest on the African margin offshore Ghana, Liberia and Côte d’Ivoire. (Illustration used with permission from Neftex.)

Because these various input data are constrained by a stratigraphic model, the geocellular models are displayed not only in true vertical depth (TVD) or two-way traveltime, but also in geologic time (above). In addition, geologists are able to project characteristics of a given strati-

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graphic interval to analogous strata in conjugate basins or in frontier areas. Geologists are also able to use qualities from a data-rich region to develop a sequence stratigraphic context for predicting facies in data-poor regions.

Plate Boundaries and Rifted and Transform Margins Plate tectonic science has established that the Earth’s outermost layer, the lithosphere, comprises a number of major and many minor plates that move relative to one another (next page).6

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This motion is driven by the convection and flow of hot ductile material in the mantle underlying the lithosphere. The lithosphere consists of two layers: the crust and the lithospheric mantle.7 The crust is further divided into two categories. Continental crust is mostly of granitic composition; its density averages about 2.7 g/cm3, and its thickness is about 35 km [22 mi] in most places but ranges from 20 to 70 km [12 to 43 mi]. Oceanic crust has a basaltic composition and is

denser and thinner than continental crust. Its density averages about 2.9 g/cm3, and its thickness ranges from 5 to 10 km [3 to 6 mi]. The higher density of the oceanic crust causes it to rest lower in the mantle than continental crust. Over geologic time, tectonic plate motions have amalgamated small continents to form supercontinents and separated them again into a collection of smaller continents distributed across the planet. The most recent giant supercontinent,

Pangea, formed during the Paleozoic era, then was rifted apart beginning about 225 to 200 million years ago [Ma]. The breakup started with Pangea separating into the Laurasia and Gondwana supercontinents in the north and south, respectively. The subsequent breakup of Laurasia and Gondwana resulted in the opening of the Atlantic and Indian oceans and evolved to the present day configuration of continents and oceans.

Eurasia plate

Eurasia plate

Juan de Fuca plate

North America plate Anatolia plate

Pacific plate Caribbean plate

Philippine plate

Africa plate Cocos plate South America plate

Arabia plate

India plate

Australia plate

Nazca plate Australia plate

Pacific plate Scotia plate Antarctica plate Antarctica plate Antarctica plate Convergent boundary barbs point to direction of convergence

Possible boundary

Major transform boundary

Divergent boundary

Plate movement

> Plates. The Earth’s lithosphere is divided into numerous plates. Relative motion of the plates (arrows) determines whether the plate boundaries are convergent, transform or divergent. [Map adapted from “Interpretative Map of Plate Tectonics,” an inset to Simkin T, Tilling RI, Vogt PR, Kirby SH, Kimberly P and Stewart DB: “This Dynamic Planet—World Map of Volcanoes, Earthquakes, Impact Craters, and Plate Tectonics,” US Geological Survey, Geologic Investigations Series Map I–2800 (2006).]

6. The lithosphere is the 50 to 200 km [30 to 120 mi] thick, rigid outer layer of Earth; its thickness is determined by the depth of the brittle-to-ductile transition temperature, which is roughly 1,000°C [1,800°F]. The upper part of the lithosphere is the crust and the lower part is the lithospheric mantle.

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For more on plate boundaries: Bird P: “An Updated Digital Model of Plate Boundaries,” Geochemistry Geophysics Geosystems 4, no. 3 (March 2003), http://dx.doi.org/10.1029/2001GC000252 (accessed August 21, 2012).

7. Earth’s mantle is the 2,900 km [1,800 mi] thick layer that lies between Earth’s crust and outer core. The mantle is divided into the upper mantle, transition zone and lower mantle. The upper mantle is about 370 km [230 mi] thick and divided into the lithospheric mantle and the asthenosphere.

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Convergent plate boundary

Transform plate boundary

Shield volcano

Island arc Trench stratovolcano tle

r

pe

Up

n ma

Divergent plate boundary

Convergent plate boundary

Oceanic spreading ridge

Continental rift zone (young plate boundary)

Trench Continental crust Uppe r ma ntle

Lithosphere Oceanic crust

Asthenosphere

Subducting plate

Hot spot Lower mantle

Plate Asthenosphere Convergent boundary

Divergent boundary

Transform boundary

> Plate boundaries. Earth’s lithospheric plates move relative to one another. This movement is accommodated along plate boundaries. Convergent boundaries occur where plates move toward one another. One plate may subduct—dive—under another; trenches mark the line of the bending, subducting plate. Chains of island arc stratovolcanoes may form along subduction zones above the downgoing plate. Transform boundaries occur where plates slide past one another; oceanic transform fault zones transfer seafloor spreading from one midocean ridge segment to another. Divergent plate boundaries occur where plates split apart at seafloor spreading ridges and continental rift zones. Hot spots occur where plumes of hot mantle material impinge on lithospheric plates; they may induce shield volcanoes and cause flood basalts to pour out over plates (not shown). [Image adapted from “Schematic Cross Section of Plate Tectonics,” an inset to Simkin T, Tilling RI, Vogt PR, Kirby SH, Kimberly P and Stewart DB: “This Dynamic Planet—World Map of Volcanoes, Earthquakes, Impact Craters, and Plate Tectonics,” US Geological Survey, Geologic Investigations Series Map I–2800 (2006).]

Midocean ridge

Plate boundary

Ocean crust

Fracture zone (inactive)

Transform fault (active part of fracture zone)

Fracture zone (inactive)

Oceanic crust Lithosphere Asthenosphere Plate boundary

> Midocean ridge and transform fault plate boundary. Midocean spreading (white and red arrows) rarely occurs along a single clean rift zone. Here, the divergent plate boundary (dashed yellow line) consists of two segments of a midocean ridge connected by a transform fault. In the transform fault, or the active part of the fracture zone between the ridge segments, the plates slide past each other in opposite directions (black opposing arrows). In the inactive part of the fracture zone, outside of the ridge segments, the plate sections are locked together and move in the same direction (black parallel arrows). (Adapted from Garrison TS: Oceanography: An Invitation to Marine Science, 4th ed. Pacific Grove, California, USA: Brooks/Cole Publishing Company, 2002.)

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The plates move relative to one another and interact with each other at their boundaries (left). The three types of plate boundaries are the following: convergent, or compressional; transform, or strike slip; and divergent, or extensional. At convergent plate boundaries, plates move toward one another. Plates respond in a number of ways when they collide, depending on whether the convergence is continent to continent, ocean to ocean or ocean to continent. Continent-tocontinent convergence—collision—results in shortening and thickening of the crust. The collision between the Indian and Asian continents is one example. This convergence created the Himalaya Mountains and Tibetan Plateau and resulted in the southeastward lateral escape of Sundaland and southeast China in the direction away from the collision between India and Asia.8 Ocean-to-ocean or ocean-to-continent convergence results in subduction: one oceanic plate dives under the other plate. An example of oceanto-ocean convergence occurs at the Marianas Trench, where the Pacific plate plunges westward under the small Philippine plate in the western Pacific Ocean. Ocean-to-continent convergence occurs along the western Andes Mountains, where the Pacific plate dives eastward under the South America plate. At transform boundaries, plates slide past each other, which occurs along the San Andreas Fault in California, USA. This fault accommodates movement of the Pacific plate northward past the North America plate. The North and East Anatolian faults in Turkey are also transform boundaries. These faults accommodate the westward movement of the Anatolia plate toward the Mediterranean Sea as it escapes the compression between the converging Eurasia and Arabia plates. At divergent plate boundaries, a plate splits, forming two smaller plates that move apart from each other. Divergent plate boundaries may start out as continental rift systems; in millions of years, these land-based rifts become oceanic rifts. Examples of modern-day continental rifts are the East African rift; the Lake Baikal rift, Russia; and the Basin and Range Province, western USA. In continental rifts, the crust undergoes extension, faulting and thinning until it splits. At the split, a volcanic ridge forms as hot mantle material wells up to fill the void left by the separating plates. The mantle material of basaltic composition accretes to the plate edges, cools and forms new oceanic crust. As the plates move apart, the oceanic crust grows, building an ocean that widens between the slowly separating plates. The process is called seafloor spreading. The Red

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Sea and Gulf of Aden rift that separates the Africa and Arabia plates is a young divergent plate boundary. The Mid-Atlantic Ridge, which encompasses the midocean rift and ridge that separates the Americas from Europe and Africa, is a mature divergent plate boundary. As continents move apart, they rarely do so along a single separation zone or rift. Rather, the rift is a series of segments offset by transform faults and fracture zones. Transform faults are strike-slip faults that connect rift segments. They transfer the spreading motion or accommodate spreading rate differences between rift segments; they are active only between rift segments.9 Transform faults leave scars on the ocean floor called fracture zones. Transform faults and fracture zones are oriented perpendicular to the midocean ridge and parallel to the spreading direction; they mark the path of plate movement as the rifted continental margins move farther apart. The ages and thermal histories of oceanic rocks differ on opposite sides of transform faults. Along the fault, younger, hotter and lower density rocks are juxtaposed against older, colder and higher density rocks. Because they are hotter, the younger rocks are thermally uplifted to a higher elevation than their older, cooler and denser cross-fault neighbors, causing a difference in ocean floor elevation on either side of the fault. These elevation differences may remain as the rocks cool, leaving scars—fracture zones. Because the fracture zones are nearly parallel to the midocean ridge spreading direction—the direction of relative plate motion—they leave tracks of the opening of the ocean (previous page, bottom). As seafloor spreading continues, previously connected continental margins move farther apart. A continental margin, where continental crust meets or transitions to oceanic crust, is a relic of faulting during continental breakup. Thus, continental margins that face a midocean rift commonly have overlaps and may also have transform and rifted margin segments. Transform margins occur where continents break up and separate by shear movement along transform strike-slip faults. Rifted margins form where continents break up and separate by extensional movement perpendicular to coastlines and along dip-slip faults. Gondwana Breakup The relative movement of adjacent tectonic plates throughout geologic time has been quantified by remote-sensing technologies. For continents, scientists determine plate movement by

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fitting apparent polar wander curves.10 For oceans, scientists determine plate movement from magnetic anomaly patterns caused by north-to-south polarity reversals of Earth’s mag-

netic field and from fracture zones on the ocean floor (below).11 However, there are no useful magnetic anomalies to constrain the Gondwana breakup history during the Cretaceous period

Magnetic chrons MC1

MC1 MC3 MC2 MC6 MC5 MC4

Is

MC2 MC3 MC4 MC5 MC6

Is

M

oc

oc

id

hr

on

s

hr

oc

ea

n

on

s

rid

ge

Normal polarity Reverse polarity

Oceanic crust

Seafloor spreading Cold and old

Lithosphere Hot and young

Plate temperature and age

> Magnetic anomalies and seafloor spreading. Scientists obtained evidence of seafloor spreading by determining the polarity of magnetic anomalies on both sides of midocean ridges. Earth’s magnetic field changes its polarity from time to time. The ocean floor is youngest and hottest at the oceanic ridge spreading center and becomes progressively older and cooler toward the continent-ocean boundary. As the ocean floor rocks and their ferromagnetic minerals cool below the Curie temperature, the ferromagnetic minerals become magnetized in the direction consistent with the existing polarity of Earth’s magnetic field. Rocks displaying dominantly normal polarity, equivalent to present-day magnetism, are shown by black stripes on the plate cross section. Rocks with dominantly reverse polarity magnetism are shown as white stripes. The symmetry of the magnetic anomaly striping on either side of the ridge demonstrates the movement of the seafloor away from the spreading center. Dating each polarity shift—normal to reverse and reverse to normal—turns the magnetic anomaly map into an magnetochronology map for seafloor spreading; the age of each reversal is an isochron (white lines)—a contour of time—and the time interval between magnetic reversals is a magnetic chron (MC), during which Earth’s magnetic field is dominantly, or constantly, one polarity.

8. Sundaland refers to the Sunda shelf region of Southeast Asia, which includes Malaysia, Sumatra, Java and Borneo. For more about the lateral escape of Southeast Asia and Sundaland: Tapponnier P, Lacassin R, Leloup PH, Schärer U, Zhong D, Wu H, Liu X, Ji S, Zhang L and Zhong J: “The Ailao Shan/Red River Metamorphic Belt: Tertiary Left-Lateral Shear Between Indochina and South China,” Nature 343, no. 6257 (February 1, 1990): 431–437. 9. Strike-slip displacement or motion refers to the horizontal movement of the other side of the fault relative to the reference side—the side on which one is standing, facing the fault. The motion is right lateral when the other side of the fault moves to the right and left lateral when the other side moves to the left. 10. For more on plate motions and polar wander: Besse J and Courtillot V: “Apparent and True Polar Wander and the Geometry of Geomagnetic Field Over the Last 200 Myr,” Journal of Geophysical Research 107, no. B11 (November 2002): EMP 6-1 to 6-31.

Besse J and Courtillot V: “Correction to ‘Apparent and True Polar Wander and the Geometry of Geomagnetic Field Over the Last 200 Myr,‘” Journal of Geophysical Research 108, no. B10 (October 2003): EMP 3-1 to 3-2. 11. For more on plate motions, magnetic anomalies and seafloor spreading: Hellinger SJ: “The Uncertainties of Finite Rotations in Plate Tectonics,” Journal of Geophysical Research 86, no. B10 (October 1981): 9312–9318. Karner GD and Gambôa LAP: “Timing and Origin of the South Atlantic Pre-Salt Sag Basins and Their Capping Evaporates,” in Schreiber BC, Lugli S and Babel M (eds): Evaporites Through Space and Time. London: The Geological Society, Special Publication 285 (January 2007): 15–35.

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Cratons Marathon FZ

Demerara Plateau

Cretaceous volcanism

AFRICA

Midocean ridge

Guinean Plateau

Aptian salt

Equatorial Segment

Romanche FZ Chain FZ Potiguar basin

Gulf of Guinea

Ascension FZ SergipeAlagoas basin Espírito Santo basin

SOUTH AMERICA

Paraná Province

Pelotas basin

Congo basin Kwanza basin

Central Segment Namibe basin

Campos basin Rio Grande FZ

Santos basin

Gabon basin

Walvis Ridge

Rio Grande Rise

Namibia basin

Tristan da Cunha hot spot Southern Segment

Rawson basin Agulhas-Falkland FZ

Falkland Segment

> Tectonic map of the South Atlantic Ocean at the end of magnetic polarity chron 34 (MC34, 84 Ma). The red line represents the midocean ridge at the end of MC34. From north to south, the South Atlantic Ocean is divided into the Equatorial, Central, Southern and Falkland segments, bounded by the Marathon, Ascension, Rio Grande and Agulhas-Falkland fracture zones (FZs). The black dots show the approximate locations of the discoveries of Tupi offshore Brazil, Azul and Cameia offshore Angola, Jubilee offshore Ghana and Zaedyus offshore French Guiana. (Adapted from Moulin et al, reference 12.)

from roughly 120 to 84 Ma because Earth’s magnetic field was stable and did not experience magnetic polarity reversals during that interval.12 Nonetheless, through dating of the flood basalts that poured over the Gondwana continent, geoscientists generally agree that the breakup of the Gondwana supercontinent, which resulted in the opening of the South Atlantic Ocean and the separation of the South America and Africa plates, started about 130 Ma during the Early Cretaceous epoch. The breakup started in the south, moved progressively north and was completed about 20 to 30 million years later during the Aptian to Albian geologic ages.13 The central

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segment opened later because the continental plate was hotter and softer there. Consequently, it stretched further and reached a higher elevation because of thermal uplift before breakup. The South Atlantic Ocean extends from the Marathon Fracture Zone (FZ) in the north to the Antarctic Plate in the south and may be divided into four segments separated by major FZs that cross the Atlantic Ocean (above). Adjacent to the Rio Grande FZ, the Rio Grande Rise and the Walvis Ridge originated from the Tristan da Cunha hot spot that is responsible for the Paraná and Etendeka flood basalts in Brazil and Namibia, respectively.14 When the ocean opened, the Rio Grande Rise and Walvis

Ridge formed as the South America plate drifted to the NW and the African plate drifted NE relative to the Tristan da Cunha hot spot. The resulting ridges formed a broad volcanic high that isolated the central segment of the South Atlantic Ocean from encroachment by marine water from the southern segment. The basin filling histories of the central and southern segments of the South Atlantic differ from one another.15 In particular, the central segment is dominated by thick salt basins that formed during the Aptian age (125 to 112 Ma), whereas the continental margins of the southern segment subsided at the margins of an open ocean.

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The equatorial South Atlantic segment began to open later in the Early Cretaceous epoch— around 112 Ma.16 In its northern latitudes, this segment encompasses the Demerara plateau of Suriname and French Guiana and the Guinea plateau in West Africa. In its southern latitudes, it includes coasts of northern Brazil, Côte d’Ivoire and Ghana.17 The opening of the equatorial segment, unlike the other segments, was not perpendicular to the continental margins because some of the plate motion was taken up by oblique movement or sideways tearing along faults.18 Geologists’ understanding of the geologic events that controlled geography, climate and basin history are based on the principles of plate tectonics. These principles form the foundation for developing exploration plays. Discoveries in the presalt and transform margin basins along the South American and western African coasts since 2006 illustrate these points. Matching Salt Basins: From Brazil to Angola The Lula oil field—renamed from Tupi in 2010 to honor former Brazilian president Luiz Inacio Lula da Silva—was discovered in 2006 within 12. Torsvik TH, Rousse S, Labails C and Smethurst MA: “A New Scheme for the Opening of the South Atlantic Ocean and the Dissection of an Aptian Salt Basin,” Geophysical Journal International 177, no. 3 (June 2009): 1315–1333. Moulin M, Aslanian D and Unternehr P: “A New Starting Point for the South and Equatorial Atlantic Ocean,” Earth-Science Reviews 98, no. 1–2 (January 2010): 1–37. Blaich OA, Faleide JI and Tsikalas F: “Crustal Breakup and Continent Ocean Transition at South Atlantic Conjugate Margins,” Journal of Geophysical Research 116, B01402 (January 2011): 1–38. Cartwright J, Swart R and Corner B: “Conjugate Margins of the South Atlantic: Namibia–Pelotas,” in Roberts DG and Bally AW (eds): Regional Geology and Tectonics: Phanerozoic Passive Margins, Cratonic Basins and Global Tectonic Maps, Vol. 1c. Amsterdam, The Netherlands: Elsevier BV (2012): 202–221. Mohriak WU and Fainstein R: “Phanerozoic Regional Geology of the Eastern Brazilian Margin,” in Roberts DG and Bally AW (eds): Regional Geology and Tectonics: Phanerozoic Passive Margins, Cratonic Basins and Global Tectonic Maps, Vol. 1c. Amsterdam, The Netherlands: Elsevier BV (2012): 222–283. 13. Szatmari P: “Habitat of Petroleum Along the South Atlantic Margins,” in Mello MR and Katz BJ (eds): Petroleum Systems of South Atlantic Margins. Tulsa: The American Association of Petroleum Geologists, AAPG Memoir 73 (2000): 69–75. 14. Hot spots are surface manifestations of mantle plumes, which are stationary thermal anomalies that produce thin upwelling conduits of magma within the mantle. Hot spot volcanism yields flood basalts and long linear chains of volcanoes within tectonic plate interiors; along each chain, the volcanoes become progressively older in the direction of plate movement. Wilson M: “Magmatism and Continental Rifting During the Opening of the South Atlantic Ocean: A Consequence of Lower Cretaceous Super-Plume Activity?,” in Storey BC, Alabaster T and Pankhurst RJ (eds): Magmatism and the Causes of Continental Break-Up. London: The Geological Society, Special Publication 68 (1992): 241–255.

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the Santos basin by Petróleo Brasileiro SA, or Petrobras.19 The discovery was made beneath Aptian salt on the Brazilian rifted margin of the central South Atlantic and established the presalt play.20 The presalt fields offshore Brazil are charged with hydrocarbons migrating from organic-rich source rocks deposited within anoxic lakes that developed around the time the South Atlantic was forming. At the start of the Aptian age, continental rifting ended and seafloor spreading began; however, lake, rather than marine, conditions prevailed as the region was uplifted above the mantle plume of the Tristan da Cunha hot spot. In these lakes above the rifted continental margins, unusual carbonates were deposited during the Early Aptian (123 to 117 Ma). Similar to the process in present-day Lake Tanganyika in East Africa, shallow lacustrine carbonates were deposited during slow deepening of the lakes. Within the Early Aptian carbonates, the fossil record shows coquina strata overlain by microbialite strata as conditions changed from fresh to hypersaline water when the climate became more arid.21 These

carbonates form the reservoirs of Brazil’s Santos and Campos presalt basins. With increased aridity during the Late Aptian (117 to 113 Ma), the basins became conducive to deposition of thick, 800- to 2,500-m [2,600- to 8,200-ft] layered evaporite sequences. Evaporites in the Santos basin show a history of rapid precipitation of mostly halite from marine waters, followed by slow precipitation of complex salts. These later salts precipitated from highly concentrated brines augmented by hydrothermal processes involving a fluid-rock chemical exchange with basaltic rock. The first 600 m [2,000 ft] of these evaporites are formed by two massive halite layers separated by a thin anhydrite layer. The top of the evaporite sequence shows a number of deposition cycles with potassium- and magnesium-rich layered evaporites.22 This entire evaporite sequence precipitated in a deep rift lake behind the barrier created by the Walvis Ridge and Rio Grande Rise. This barrier was penetrated by deep fissures along which marine waters traveled, interacting chemically with the basaltic wall rock and leaking into the evaporating lake.

Quirk DG, Hertle M, Jeppesen JW, Raven M, Mohriak W, Kann DJ, Nørgaard M, Mendes MP, Hsu D, Howe MJ and Coffey B: “Rifting, Subsidence and Continental Break-Up Above a Mantle Plume in the Central South Atlantic,” in Mohriak WU, Danforth A, Post PJ, Brown DE, Tari GC, Nemc˘ok M and Sinha ST (eds): Conjugate Divergent Margins. London: The Geological Society, Special Publication 369 (in press). 15. Séranne M and Anka Z: “South Atlantic Continental Margins of Africa: A Comparison of the Tectonic vs. Climate Interplay on the Evolution of Equatorial West Africa and SW Africa Margins,” Journal of African Earth Sciences 43, no. 1–3 (October 2005): 283–300. 16. Moulin et al, reference 12. 17. The Guyanas, or Guianas, is the region of northern South America that includes the nations of Suriname, Guyana and French Guiana. West Africa, or western Africa, is the westernmost region of the African continent and its southern edge extends along the northern coastline of the Gulf of Guinea and includes, from east to west, Nigeria, Togo, Benin, Ghana, Côte d’Ivoire, Liberia, Sierra Leone and Guinea. 18. Darros de Matos RM: “Tectonic Evolution of the Equatorial South Atlantic,” in Mohriak W and Talwani M (eds): Atlantic Rifts and Continental Margins. Washington, DC: American Geophysical Union, Geophysical Monograph 115 (2000): 331–354. Mascle J, Lohman P, Clift P and ODP 159 Scientific Party: “Development of a Passive Transform Margin: Côte d’Ivoire–Ghana Transform Margin—ODP Leg 159 Preliminary Results,” Geo-Marine Letters 17, no. 1 (February 1997): 4–11. Darros de Matos RM: “Petroleum Systems Related to the Equatorial Transform Margin: Brazilian and West African Conjugate Basins,” in Post P, Rosen N, Olson D, Palmes SL, Lyons KT and Newton GB (eds): Petroleum Systems of Divergent Continental Margin Basins. Tulsa: Gulf Coast Section, Society for Sedimentary Geology (2005): 807–831. 19. Beasley CJ, Fiduk JC, Bize E, Boyd A, Frydman M, Zerilli A, Dribus JR, Moreira JLP and Pinto ACC: “Brazil’s Presalt Play,” Oilfield Review 22, no. 3 (Autumn 2010): 28–37.

20. Presalt refers to before the formation or deposition of salt deposits. Presalt reservoirs are beneath salt deposits that have not flowed away from their place of deposition—beneath the autochthonous, or in place, salt. This definition differentiates presalt strata from subsalt or postsalt strata. For more: Beasley et al, reference 19. 21. Coquina is a limestone formed principally from shell fragments and indicates a nearshore environment with vigorous wave action. Microbialites, which are carbonate structures thought to be formed by microbes, have a range of shapes and sizes. They form in environments that are not conducive to the growth of corals. 22. Hardie LA: “On the Significance of Evaporites,” Annual Review of Earth and Planetary Sciences 19 (May 1991): 131–168. Jackson MPA, Cramez C and Fonck J-M: “Role of Subaerial Volcanic Rocks and Mantle Plumes in Creation of South Atlantic Margins: Implications for Salt Tectonics and Source Rocks,” Marine and Petroleum Geology 17, no. 4 (April 2000): 477–498. Nunn JA and Harris NB: “Subsurface Seepage of Seawater Across a Barrier: A Source of Water and Salt to Peripheral Salt Basins,” Geological Society of America Bulletin 119, no. 9–10 (September–October 2007): 1201–1217. Nunn JA and Harris NB: “Erratum for ‘Subsurface Seepage of Seawater Across a Barrier: A Source of Water and Salt to Peripheral Salt Basins,’” Geological Society of America Bulletin 120, no. 1–2 (January– February 2008): 256.

47

E

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>Seismic lines across conjugate presalt rifted margins. These paired seismic lines are dip lines from the Santos basin offshore Brazil (above) and the Kwanza basin offshore Angola (next page, top). The Santos basin seismic section is from a generic 2D seismic line crossing close to the Lula field, a presalt discovery. The seismic section shows a nearly 2-km [1.2-mi] thickness of presalt sediments underneath the salt. The Kwanza basin section, offshore Angola, is from a 3D seismic survey and shows a well-developed presalt section separated from postsalt sediments by complex salt geometries. (The Santos basin section is used with permission from WesternGeco and TGS. The Kwanza basin section is used with permission from WesternGeco and Sonangol.)

The necessary factors promoting such thick salt accumulations were a rapidly sinking margin with balance-filled basins or lakes behind an elevated outer volcanic high. This volcanic high was a leaky barrier that restricted inflow of seawater in an environment characterized by a warm, arid, desert climate (next page, bottom).23 Conditions were somewhat similar to present-day conditions in the Dead Sea basin and in the Danakil Depression on the Afar Peninsula, northeast Africa.24 These layered salts form the seal for the presalt reservoirs (See “Salt Deposition in Actively Spreading Basins,” page 50).

The end of the Aptian age saw the final breaching of the Walvis Ridge–Rio Grande Rise barrier accompanied by flooding of marine waters from the southern segment of the South Atlantic Ocean. These open marine conditions allowed ocean waters to fill the basins of the central segment, halting any further evaporite deposition. Marine sediments formed on top of the salt, starting with marine carbonates in the Albian age (113 to 110 Ma). The postsalt sedimentation was controlled by continual opening and deepening of the South Atlantic by global changes of sea level. As the ocean opened, the

rifted margins tilted seaward, causing halokinesis, in which the salt flows and deforms, giving rise to the salt structures that affected postsalt sediments where large volumes of oil were found in the Campos basin (above).25 The Tupi discovery in 2006 opened up a new petroleum play in the central South Atlantic, the presalt play. Lula field lies in 2,126 m [6,975 ft] of water in the Santos basin Block BM-S-11 about 250 km [155 mi] southeast of Rio de Janeiro. The 1-RJS-628A discovery well was drilled to 4,895 m [16,060 ft] TVD subsea.26 The well flowed 780 m3/d [4,900 bbl/d] of oil and 187,000 m3/d [6.6 MMcf/d] (continued on page 52)

23. Davison I: “Geology and Tectonics of the South Atlantic Brazilian Salt Basins,” in Ries AC, Butler RWH and Graham RH (eds): Deformation of the Continental Crust: The Legacy of Mike Coward. London: The Geological Society, Special Publication 272 (January 2007): 345–359. Lakes or basins are balance filled when the rate of water and sediment input is similar to the rate that the accommodation space—area and depth—forms. For more: Carroll AR and Bohacs KM: “Stratigraphic Classification of Ancient Lakes: Balancing Tectonic and Climatic Controls,” Geology 27, no. 2 (February 1999): 99–102. 24. Montaron B and Tapponnier P: “A Quantitative Model for Salt Deposition in Actively Spreading Basins,” Search and Discovery Article 30117, adapted from an oral presentation at the AAPG International Conference and

48

Exhibition, Rio de Janeiro, November 15–18, 2009. Bosworth W, Huchon P and McClay K: “The Red Sea and Gulf of Aden Basins,” Journal of African Earth Sciences 43, no. 1–3 (October 2005): 334–378. Mohriak WU and Leroy S: “Architecture of Rifted Continental Margins and Break-Up Evolution: Insights from the South Atlantic, North Atlantic and Red Sea– Gulf of Aden Conjugate Margins,” in Mohriak WU, Danforth A, Post PJ, Brown DE, Tari GC, Nemc˘ok M and Sinha ST (eds): Conjugate Divergent Margins. London: The Geological Society, Special Publication 369, http:// dx.doi.org/10.1144/SP369.17 (accessed September 17, 2012). 25. Halokinesis is the deformation of salt. Halokinetic processes include downslope movement under gravity flow, expulsion and diapirism caused by overburden loading and faulting resulting from tectonic stretching or

shortening. Salt deformation may cause deformation in the strata deposited above it. Hudec MR and Jackson MPA: “Terra Infirma: Understanding Salt Tectonics,” Earth-Science Reviews 82, no. 1–2 (May 2007): 1–28. Quirk DG, Schødt N, Lassen B, Ings SJ, Hsu D, Hirsch KK and Von Nicolai C: “Salt Tectonics on Passive Margins: Examples from Santos, Campos and Kwanza Basins,” in Alsop GI, Archer SG, Hartley AJ, Grant NT and Hodgkinson R (eds): Salt Tectonics, Sediments and Prospectivity. London: The Geological Society, Special Publication 363 (January 2012): 207–244. Beasley et al, reference 19. 26. Parshall J: “Presalt Propels Brazil into Oil’s Front Ranks,” Journal of Petroleum Technology 62, no. 4 (April 2010): 40–44.

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> Conditions conducive for thick salt accumulations. By the Aptian, about 120 Ma, the South Atlantic Ocean (map, center) had scissored open from the south. The central segment of the South Atlantic was isolated from the open marine conditions of the southern segment by the Walvis Ridge (purple). The region was in an arid belt (between dashed white lines) where climate conditions were similar to those in the present-day Atacama desert, northern Chile (bottom left ), and Kalahari desert, southern Africa (bottom right ). The central segment contained balance-filled basins and lakes. Under these climatic and isolated basin conditions, the basins and lakes became centers for precipitation of thick, layered salt sequences from basinal and hydrothermal brines, which were fed by marine water flowing through fractures in the leaky basaltic dam formed by the Walvis Ridge. (Map courtesy of CR Scotese, used with permission.)

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49

Salt Deposition in Actively Spreading Basins

Rifting, Spreading and Tectonics The salt basins that face one another between the Rio Grande Rise and the Gulf of Guinea are among the largest found along Phanerozoic passive ocean margins (below). They formed during the Aptian (125 to 110 Ma), during the opening stages of the central South Atlantic. The geometric, kinematic and temporal environment of this lower Cretaceous salt deposition appears strikingly similar to that of the Mid-Late Miocene Red Sea (15 to 5 Ma).1 After the Tristan da Cunha hot spot induced giant volcanic eruptions that covered huge areas of the African–South American lithosphere with thick flood basalts about 143 Ma, the plates started to separate slowly at several millimeters per year. Narrow rifts, 50 to 80 km [31 to 50 mi] wide, which overlapped, formed

Tra n

sfo

rm

along the newborn plate boundary. Basaltic volcanism and anoxic deepwater lakes—some deeper than 1,000 m [3,300 ft], similar to Lake Tanganyika today—punctuated the geology of such rifts in the Late Hauterivian to Early Barremian (133 to 128 Ma).2 Continental separation was completed 128 to 125 Ma. As full seafloor spreading began, the rate of plate separation increased to a few centimeters per year. The marine basin, now 1,700 km [1,060 mi] long, 300 to 500 km [190 to 310 mi] wide and 2 km [1.2 mi] deep, remained isolated between two large “dams” formed by the nascent equatorial Atlantic transform margin to the north and the Walvis Ridge and Rio Grande Rise to the south. These dams restricted seawater flow into the basin—flow that took place mostly along tectonic fissures through the southern

ma

rgin AFRICA

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Hot spot

> South Atlantic restoration. The Aptian, about 120 Ma, salt basin (purple) was 1,700 km [1,060 mi] long and restricted from open ocean conditions by the Tristan da Cunha hot spot (red circle) to its south and the embryonic equatorial Atlantic transform margin (opposing red arrows) to its north. The black arrows indicate the direction of plate movement. (Map courtesy of CR Scotese, used with permission.)

50

Walvis Ridge. Rapid evaporation of seawater created thick, layered evaporite deposits. Continuous open marine conditions were reestablished in the Early Albian (112 to 110 Ma).

Evaporites in the Santos Basin Three conditions are required to create a thick, layered salt deposit: a basin about 1,500 m [4,900 ft] deep, a continuous supply of mineral-laden seawater and a warm and arid climate. As evaporation takes place, the basin water level drops quickly and stabilizes to a critical level: The evaporation rate equals the water intake rate. The water salinity increases gradually until the saturation concentration is reached for the least soluble salt mineral contained in the water. Layers of calcite, dolomite and gypsum precipitate—in that order—followed by halite (rock salt). Halite precipitates in quantities just sufficient to maintain the water salinity at the halite saturation level; this process can last several thousand years to accumulate hundreds of meters of halite. If the climate becomes wetter, increased freshwater intake from rivers and rain may reduce the salinity enough to stop halite precipitation. For example, salinity may drop back to the gypsum precipitation point and eventually increase back to the halite precipitation point. This is the layered sequence observed in the bottom 600 m [2,000 ft] of Santos basin evaporites. Water salinity levels may increase further, until they reach the saturation point at which complex salts begin to precipitate. These salts are potassium-, calcium- and magnesium-rich evaporites such as sylvite, carnallite and tachyhydrite. Precipitation of complex salts requires an extremely arid climate and precipitation may take a long time because these highly saline brines evaporate very slowly. During this process, the lake surface level will not change despite salt accumulating on the lake bottom. The final result is a salt flat (next page).

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Freshwater lakes form.

Freshwater lakes deepen. Ocean level falls.

Ocean level rises, spills over barrier and floods into freshwater lakes.

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Basin level drops as water evaporates.

6 Salt deposition starts.

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Salt deposition ending. Terminal brine marks final salt deposition.

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Ocean level falls. Fractured ridge allows hydraulic communication between ocean and lake.

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> Salt deposition sequence. During early rifting (1), freshwater lakes form on the stretching continental margin. (The developing ocean is on the left side of each panel.) The ocean level drops and the lakes deepen (2) as the stretching continental margins thin and subside. The barrier that separates the ocean from the lakes increases in relief with respect to the lake bottom. Sea level rises (3), and seawater spills over the barrier and mixes with the lake water. About 123 Ma in the Early Aptian (4), sea level falls by 50 m [80 ft] and isolates the basins from open ocean waters. The evaporation rate from the basins (5) is greater than the rate of water influx from rivers and rainfall and from seawater springs emanating from the leaky barrier; such leaks are the result of fractures and fissures. The basin water level drops and water salinity gradually increases until the brine salinity level reaches the saturation concentration of the least soluble chemical component in the brine, which begins to deposit as a salt mineral (white, 6). During salt deposition, salt layers (not shown) form as the brine chemistry changes. Salinity and salt saturation concentrations depend on the climatic water balance within the basins and the seawater input to them through the leaky barrier. Salt mineral precipitation begins with the least soluble chemical component in the brine. This component precipitates until it depletes. More soluble components precipitate later. In this way, salt layers gradually build up and fill the basins to form thick layered salt sequences. The final episode of salt deposition is marked by a terminal brine (purple, 7) of high salinity, supersaturated with the least soluble component at the time. Finally, sea level rises sufficiently to inundate the continental margins (8); open marine conditions are reestablished above the salt basins and such marine conditions shut down salt deposition.

1. Mohriak WU and Leroy S: “Architecture of Rifted Continental Margins and Break-Up Evolution: Insights from the South Atlantic, North Atlantic and Red Sea– Gulf of Aden Conjugate Margins,” in Mohriak WU, Danforth A, Post PJ, Brown DE, Tari GC, Nemc˘ok M and Sinha ST (eds): Conjugate Divergent Margins. London: The Geological Society, Special Publication 369, http://dx.doi.org/10.1144/SP369.17 (accessed September 17, 2012). Bosworth W, Huchon P and McClay K: “The Red Sea and Gulf of Aden Basins,” Journal of African Earth Sciences 43, no. 1–3 (October 2005): 334–378. 2. Karner GD and Gambôa LAP: “Timing and Origin of the South Atlantic Pre-Salt Sag Basins and Their Capping Evaporates,” in Schreiber BC, Lugli S and Ba˛bel M (eds): Evaporites Through Space and Time. London: The Geological Society, Special Publication 285 (January 2007): 15–35.

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3. 4.

5. 6.

Montaron B and Tapponnier P: “A Quantitative Model for Salt Deposition in Actively Spreading Basins,” Search and Discovery Article 30117, adapted from an oral presentation at the AAPG International Conference and Exhibition, Rio de Janeiro, November 15–18, 2009. Montaron and Tapponnier, reference 2. Hardie LA: “The Roles of Rifting and Hydrothermal CaCl2 Brines in the Origin of Potash Evaporites: An Hypothesis,” American Journal of Science 290, no. 1 (January 1990): 43–106. Hardie LA: “On the Significance of Evaporites,” Annual Review of Earth and Planetary Sciences 19 (May 1991): 131–168. Warren JK: Evaporites: Sediments, Resources and Hydrocarbons. Berlin: Springer-Verlag, 2006. Montaron and Tapponnier, reference 2. Montaron and Tapponnier, reference 2.

During the Aptian, South Atlantic salt basins were located at latitudes corresponding to the arid belt that contains most of the southern hemisphere’s modern deserts. The initial evaporation rate was probably 2 m [7 ft] per year greater than the rainfall input, a rate currently observed in the Red Sea.4 At an average halite deposition rate of 2 to 3 cm [0.8 to 1.2 in.] per year, it may have taken 20,000 to 30,000 years to deposit the lowermost 600 m of Santos basin evaporites.5 Above that level, there are at least nine cycles containing complex salts, and these could have taken 10 times longer to precipitate. Replacing water by salt doubles the weight applied to the basin floor and accelerates subsidence. Approximately 30% of accommodation space is gained in about 50,000 years by adding 500 m [1,600 ft] to the initial 1,500-m [4,900-ft] basin depth. Observations from modern analogs such as Lake Assal in the Afar region, Ethiopia, suggest seawater entered the salt basin through fissures across the basaltic Walvis Ridge. This fissural process is also based on other considerations: • The volumetric flow rate through cracks must be small, as required by the salt precipitation model. • Because fissures in basalts can be up to a hundred meters deep, seawater flowing through fissures is less sensitive to variations in ocean water level compared to that required by flow over a dam. • When the evaporation rate increases and the basin level drops below the ocean level, the hydraulic-head difference will tend to promote flow through the fissures to maintain the basin’s water level. • The fractures provide a large contact surface between seawater and basalts, which favors the rock-to-fluid chemical exchange required for a chemical composition that is compatible with complex salt deposition.6 Field observations and model results demonstrate that the deposition of thick, layered evaporitic sequences requires a deep basin in a hot and arid climate with a continuous supply of mineral-laden saltwater. These conditions must remain stable long enough for thick deposits to accumulate.

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of gas on a 5/8-in. choke, producing light oil with a density of about 880 kg/m3 [30° API gravity] and a low sulfur content of about 0.5%.27 Development drilling in the field confirmed the operator’s estimates of up to 1,000 million m3 [6.5 billion bbl] of recoverable oil, thus drawing worldwide attention to Brazil’s presalt play.28 Many subsequent presalt discoveries have been made in the Santos and Campos basins of Brazil.

In 2012, the Azul-1 well by Maersk Oil and then the Cameia-1 well by Cobalt International Energy, Inc., extended the proven presalt play across the South Atlantic to the Kwanza basin, offshore Angola.29 The Azul-1 well was in 953 m [3,130 ft] of water in Kwanza basin Block 23; the well was drilled to 5,334 m [17,500 ft] and demonstrated potential flow capacity of greater than 3,000 bbl/d [480 m3/d] of oil. The Cameia-1 well

A FR ICA Angola 20 21 Lontra

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> Kwanza basin presalt prospects and discoveries. The Cobalt Cameia-1 and Cameia-2 wells discovered and appraised, respectively, oil reservoirs in the synrift (light brown) and postrift (yellow) sedimentary basins under the autochthonous salt (purple)—the presalt sediments—in Block 21 (center right ), Kwanza basin offshore Angola. Cobalt plans to drill the Lontra, Idared, Mavinga and Bicuar wells (dashed lines) to test other prospects in Blocks 20 and 21. The Cameia-1 well discovered a superpay reservoir (bright green) atop a basement high (bottom). Cobalt drilled the Cameia-2 well, a step-out well, to confirm the size of the discovery and to explore prospective reservoir zones below the superpay reservoir. The appraisal well confirmed the discovery and underlying reservoir intervals (light green), which are separated by sealing intervals (red). (Illustrations used with permission from Cobalt International Energy, Inc., reference 32.)

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was in 1,682 m [5,518 ft] of water in Kwanza basin Block 21; the well was drilled to 4,886 m [16,030 ft] and flowed 5,010 bbl/d [800 m3/d] of oil and 14.3 MMcf/d [405,000 m3/d] of gas. In the process leading up to the Cameia-1 discovery, exploration experts at Cobalt International Energy recognized that during the Aptian age, the present-day Kwanza and Campos presalt basins were in the same depositional basin, separated by only 80 to 160 km [50 to 100 mi]; explorationists concluded the basins must have shared the same presalt history and have similar characteristics.30 The presalt play that led to the Tupi discovery in the Brazilian Santos basin was extended north along the Brazilian coastline to the Campos basin. Cobalt drilled the Cameia-1 well to hunt for a Campos basin presalt play analog across the Atlantic Ocean in the Kwanza basin offshore Angola. The Cameia-1 oil discovery well drilled into a reservoir that contained high-quality, highly permeable and fractured carbonates in postrift and presalt strata atop a basement high and was sealed by salt. The well encountered an oil column that was about 370 m [1,200 ft] thick and contained more than 270 m [900 ft] of net pay.31 To appraise the discovery, Cobalt drilled the Cameia-2 well and confirmed the vertical and lateral extent, geometry and quality of the reservoir (left). The appraisal well validated the Cobalt model of additional reservoirs within the postrift and synrift strata beneath the original discovery and indicated the reservoirs were separated by seals. Cobalt is conducting ongoing testing to determine reservoir potential—the number of reservoirs and seals, how the fluids vary between the reservoirs, the reservoir properties and the depths to the oil/water contacts.32 Matching Turbidite Sequences: From Ghana to French Guiana The West Cape Three Points partnership discovered the Jubilee oil field offshore Ghana in June 2007. The partnership comprises Kosmos Energy Ltd., Tullow Oil plc, Anadarko Petroleum Corporation, Sabre Oil & Gas, Inc., Ghana National Petroleum Company and EO Group Ltd. The Mahogany-1 discovery well encountered 90 m [300 ft] of high-quality pay in an upper Cretaceous turbidite reservoir confined by a combination structural-stratigraphic trap.33 In August 2007, the Hyedua-1 well, located 5.3 km [3.3 mi] southwest of the Mahogany-1 discovery, encountered 41 m [130 ft] of high-quality reservoir in equivalent turbidite sandstones. These wells opened up a deep-

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Present-day 2,000-m [6,560-ft] isobath

Zaedyus discovery, Guyane Maritime, French Guiana Jubilee discovery, Tano basin, Ghana

> Opening of the equatorial Atlantic Ocean. Rifting between northern South America and southern West Africa started during the Early Cretaceous about 125 Ma (top left). Small basins opened when continental crust stretched, thinned and faulted. These basins filled with sediment from the eroding continental uplands and were deformed along the transform fault zones. During the Late Aptian to Early Albian, about 110 Ma (bottom left), oceanic spreading and accretion began. Ocean floors grew as the plates were separating during the Late Albian, about 100 Ma (top right). By Late Santonian to Early Campanian, about 85 Ma (bottom right), the continental separation was complete. The seafloor spreading and passive margin phase began and the steep transform margins subsided thermally and were cut, loaded and blanketed by river and delta sediments from the continents while South America and Africa continued to separate. (Adapted from Brownfield ME and Charpentier RR: “Geology and Total Petroleum Systems of the Gulf of Guinea Province of West Africa,“ Reston, Virginia, USA: US Geological Survey Bulletin 2207-C, 2006.)

water play targeting reservoirs in Late Cretaceous turbidites along the equatorial African transform margin, which stretches from northern Sierra Leone east to southern Gabon in the equatorial segment of the South Atlantic Ocean. Deepwater turbidite fields discovered offshore Ghana are charged with hydrocarbons sourced from organic-rich sediments that rapidly filled deep, active pull-apart basins during the Early Cretaceous epoch (above). These basins formed on rifted continental crust between transform faults. During the Albian age, the continents split and seafloor spreading began. Oblique motion between the two margins is recorded by transform faults and fracture

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27. “BG, Petrobras Announce Discovery of Oil Field in Santos Basin Offshore Brazil,” Drilling Contractor 62, no. 6 (November–December 2006): 8. 28. “Country Analysis Briefs: Brazil,” US Energy Information Administration (February 28, 2012), http://www.eia.gov/ countries/cab.cfm?fips=BR (accessed August 29, 2012). 29. “Maersk Oil Strikes Oil with Its First Pre-Salt Well in Angola,” Maersk Oil (January 4, 2012), http://www. maerskoil.com/Media/NewsAndPress Releases/Pages/MaerskOilstrikesoilwithitsfirst pre-saltwellinAngola.aspx (accessed March 29, 2012). “Cobalt International Energy, Inc. Announces Successful Pre-Salt Flow Test Offshore Angola,” Cobalt International Energy, Inc. (February 9, 2012), http://ir.cobaltintl.com/phoenix.zhtml?c=231838&p= irol-newsArticle&ID=1659328&highlight (accessed April 4, 2012). 30. Cobalt International Energy, Inc.: “Update on West Africa and Gulf of Mexico Drilling Programs,” (February 8, 2012), http://phx.corporate-ir.net/External.File?item= UGFyZW50SUQ9MTI1NzQyfENoaWxkSUQ9LTF8VHlwZT0 z&t=1 (accessed August 2, 2012). Dribus JR: “Integrating New Seismic Technology and Regional Basin Geology Now a Must,” Journal of Petroleum Technology 64, no. 10 (October 2012): 84–87.

31. Cobalt International Energy, Inc.: “Investor Presentation—March 2012,” (March 13, 2012), http://phx.corporate-ir.net/phoenix.zhtml?c=231838&p= irol-presentations (accessed June 8, 2012). 32. “Multiple Catalysts To Grow Shareholder Value,” Cobalt International Energy, Inc. (September 19, 2012), http:// phx.corporate-ir.net/External.File?item=UGFyZW50SUQ9 NDgwMTA3fENoaWxkSUQ9NTEzNzk4fFR5cGU9MQ== &t=1 (accessed September 20, 2012). 33. A turbidite is a rock deposited from a turbidity flow, which is an underwater current of sediment-laden water that moves rapidly down a slope. The gravity, or density, current moves downslope because its density is higher than that of the surrounding water. Dailly P, Henderson T, Hudgens E, Kanschat K and Lowry P: “Exploration for Cretaceous Stratigraphic Traps in the Gulf of Guinea, West Africa and the Discovery of the Jubilee Field: A Play Opening Discovery in the Tano Basin, Offshore Ghana,” in Mohriak WU, Danforth A, Post PJ, Brown DE, Tari GC, Nemc˘ok M and Sinha ST (eds): Conjugate Divergent Margins. London: The Geological Society, Special Publication 369, http://dx.doi. org/10.1144/SP369.12 (accessed August 7, 2012).

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Offset, km

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Suriname–French Guiana abyssal plain

> Conjugate transform margins. These seismic lines cross the Suriname–French Guiana (above) and Côte d’Ivoire–Ghana (next page, top) transform margins; the red dots on the globes are the locations of these seismic sections. The red lines mark the approximate position of the Demerara Fracture Zone (FZ) and the Romanche FZ, on the left and right, respectively. Transform margins are characterized by shallow dipping, often narrow, continental margins, bordered by marginal ridges that backstop steep continental slopes across abrupt continent-ocean boundaries leading to oceanic abyssal plains. Explorers are targeting reservoirs located in abyssal plain sediments in upper Cretaceous turbidites that lie on top of lower Cretaceous organic-rich source rocks. The green dots mark the approximate stratigraphic position of these upper Cretaceous reservoirs. These Cretaceous source and reservoir rocks are sealed and buried under marine shales. On the Côte d’Ivoire–Ghana seismic line, the labels A through F represent stratigraphic units identified from seismic data. [Adapted from Greenroyd CJ, Peirce C, Rodger M, Watts AB and Hobbs RW: “Demerara Plateau—The Structure and Evolution of a Transform Passive Margin,” Geophysical Journal International 172, no. 2 (February 2008): 549–564.]

zones, and subsidence and sediment deposition occurred during rifting and subsequent sag of the margins (above). The opening and deepening of the equatorial South Atlantic and the global rise and fall of sea level controlled sedimentation after continental breakup. Erosion of the continent led to deposition of sediments in deltas on the continental margins. When sea level fell—a lowstand—the rivers cut through their deltas and carried sediments, often in sediment avalanches known as turbidity currents, onto the steep continental slopes and toward the deep abyssal plain. Sands that were deposited as these turbidity currents slowed may have formed reservoirs for deepwater oil fields such as those of the upper Cretaceous series in the Jubilee field. Subsequent deposition of muds sealed these reservoirs as they were

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buried beneath thousands of meters of younger sediment. During the Late Cretaceous epoch, the movement of the tectonic plates changed direction, causing deformation of the rifted margin and the formation of structures that helped form traps, and oil started migrating updip toward the coast (next page, bottom right).34 The partnership drilled the Mahogany-1 well to reservoir rock in a Turonian-stage stack of lowstand turbidite sands on the SW flank of the South Tano ridge.35 The reservoir was 3,530 to 3,760 m [11,600 to 12,300 ft] below the seafloor. A drillstem test demonstrated that the well was capable of flowing oil at 20,000 bbl/d [3,200 m3/d]. The oil was sourced from Early Cretaceous rift-related organic-rich shales. The Jubilee well proved the Late Cretaceous turbi-

dite play concept and subsequent drilling revealed that Jubilee is part of a collection of fields offshore Ghana that includes Tweneboa, Enyenra and Ntomme. Similar Late Cretaceous turbidite reservoirs occur along the entire equatorial African coast, which have led to additional oil discoveries such as the Akasa and Teak fields offshore Ghana, the Paon field offshore Côte d’Ivoire and the Venus, Mercury and Jupiter fields offshore Sierra Leone. Tullow Oil sought to project the Jubilee play to the transform margin of South America and duplicate the company’s deepwater success.36 Exploration experts at Tullow Oil used the principles of plate tectonics, followed the major fracture zones across the equatorial Atlantic and identified basins offshore South America that displayed similar elements of the Jubilee play.

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They found evidence for an upper Cretaceous series of lowstand turbidite channels and fans deposited during seafloor spreading and buried under a thick sequence of marine shales. They inferred the presence of Cretaceous source rocks and stratigraphic traps, buried and sealed by the marine shales. This led the exploration teams to focus on the continental slope off the Guyana 34. Antobreh AA, Faleide JI, Tsikalas F and Planke S: “Rift–Shear Architecture and Tectonic Development of the Ghana Margin Deduced from Multichannel Seismic Reflection and Potential Field Data,” Marine and Petroleum Geology 26, no. 3 (March 2009): 345–368. 35. Dailly et al, reference 33. 36. Patel T: “Did the Continental Drift Create an Oil Bonanza?: Tullow Oil Bets Huge Fields Are ‘Mirrored’ Across the Atlantic,” Bloomberg Businessweek (February 24, 2011), http://www.businessweek.com/ magazine/content/11_10/b4218020773519.htm (accessed August 20, 2012).

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Midfan channelized lobes

Inner fan channels

Canyon fed by active nearshore littoral drift or relict shelf sands

Slump scar Inner fan Midfan channelized and unchannelized sands

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500 to 2,000 m [1,640 to 6,562 ft]

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10 to 50 km 5.4 to 27 mi Basin plain

> Reservoirs in Late Cretaceous turbidites. Explorationists looked for canyons feeding reservoir rocks in channel-levee and turbidite fan deposits on the basin floor that originated from the Guyana Continental Shelf and slope. These reservoir rocks are sourced and charged by Early Cretaceous organic-rich shales that were deposited during continental rifting. Since their deposition, these reservoir rocks have been buried and sealed by marine shales (not shown). Expected well log responses are plotted for the five types of deposits (boxed red areas between black curves); the left curve is spontaneous potential or gamma ray, and the right curve is resistivity. (Illustration used with permission from Tullow Oil plc.)

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Shelf and east of the Demerara Plateau offshore French Guiana (below).37 Tullow Oil and partners acquired 2,500 km2 [970 mi2] of high-quality 3D marine seismic data over the steep continental slope offshore French Guiana.38 Explorers at Tullow Oil used these data to look for submarine canyons and turbidite deposits on the basin floor that originated from the Guyana Continental Shelf and slope. These seismic data showed features similar to those observed in 3D seismic data over the Jubilee field offshore Ghana. The exploration team identified and mapped a number of prospects (next page). After follow-up regional investigations, the Tullow Oil team decided to test the play by drilling a well at the GM-ES-1 location within the Zaedyus prospect, in the Guyane Maritime license, which is about 150 km [93 mi] offshore.39

Tullow Oil started operations in March 2011, drilling near the toe of the continental slope in 2,048 m [6,719 ft] of water. By September 2011, the company announced the discovery of 72 m [240 ft] of net oil pay within two turbidite fans.40 Wireline logs and samples of reservoir fluids showed good quality reservoir sands at a reservoir depth of 5,711 m [18,740 ft]. The Zaedyus exploration well proved that the Jubilee play— developed for the transform margin offshore Ghana and applied successfully elsewhere along the equatorial African margin—was also applicable to the transform margin offshore French Guiana and probably elsewhere along the transform margin of northern South America.

Learning from Success The recent history of oil discovery along the South Atlantic margins has been one of learning from success. Pioneering explorationists studied the large discoveries of the Lula reservoir in the Santos basin, offshore Brazil, and the Jubilee reservoir, offshore Ghana, and stepped along the same margin to look across the ocean where conjugate margins hosted similar large discoveries. Explorationists used the principles of plate tectonics to leverage their accomplishments. When a continent splits and a new spreading center opens up, plate tectonic concepts provide the basis for hypothesizing which series of tectonic and stratigraphic events will occur. Armed with the principles of plate tectonics and astute observations from exploration plays that have led to successful discoveries, explorationists have extrapolated plays into new leads,

Oil discovery Gas condensate and oil discovery Prospect Dry hole Oil shows

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> Extending West African success across to South America. Tullow Oil plc used plate tectonic concepts to develop an exploration program to extend the Jubilee play (black star) proved along the West Africa transform margin to the northern South America transform margin. The transform margins (gray shading) on the west and east sides of the Equatorial Atlantic have similar geology. Explorationists had recognized Late Cretaceous stratigraphic traps within the Guyana-Suriname basin that were analogous to those proved by the Jubilee and similar discoveries in West Africa. Tullow explorationists made the Zaedyus discovery in the Guyane Maritime license, offshore French Guiana (red star). (Illustration adapted with permission from Tullow Oil plc.)

Oilfield Review

Seismic horizon relationship View angle Structural high

Turbidite feeder canyon Late Cretaceous horizon

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> Jubilee analogs offshore French Guiana. Tullow Oil plc acquired 2,500 km2 [970 mi2] of 3D seismic data in 2009 (red box in map inset). The depth-based seismic interpretation image (top), viewed from above and the northeast, shows an Early Cretaceous horizon (color-coded in red to blue from shallow to deep) overlain by a Late Cretaceous horizon (brown to yellow) intersecting at the steep continental slope formed by the transform margin. The data revealed features similar to those observed in the Tano–West Cape Three Points area, offshore Ghana. These features include a turbidite feeder canyon and structural high that focus sediments into channels and fan systems that are prospects for reservoirs. The close-up view of the area (bottom) shows channels and turbidite fans imaged by the 3D seismic data. (Images used with permission from Tullow Oil plc.)

37. Plunkett J: “French Guiana—A New Oil Province,” presented at the Kayenn Mining Symposium, Cayenne, French Guiana, December 1–3, 2011. 38. The partnership was a joint venture between Tullow Oil plc—the operator—Royal Dutch Shell, Total and Northpet, a company owned 50% by Northern Petroleum plc and 50% by Wessex Exploration plc. Royal Dutch Shell formally took over as operator of the Guyane Maritime license on February 1, 2012. 39. Plunkett, reference 37. 40. “Zaedyus Exploration Well Makes Oil Discovery Offshore French Guiana,” Tullow Oil plc (September 9, 2011), http://www.tullowoil.com/index.asp?pageid= 137&newsid=710 (accessed August 10, 2012).

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prospects and drilling targets both regionally and globally. Understanding plate tectonics also allows explorationists to take what they learn from one play and ask, “What if?” If hydrocarbons are found in an immature rift margin setting, could one find the same in a mature rift margin or a transform margin setting? In recent years, exploration companies have answered these questions affirmatively through discovery wells. Recent discoveries in the Albert rift basin of Uganda, the East Africa

rift basin of Kenya, the Levant basin offshore Israel and Cyprus and the Mozambique basin offshore Tanzania have been similarly impressive. Plate tectonic concepts and models, and their ability to engender reasoned hypotheses for new plays, are powerful exploration tools for hitherto undeveloped basins. They are also cause for reexamining basins that have been explored but deemed either hydrocarbon poor or too risky to develop. —RCNH

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