PETR 571 Week 2‐notes
Weathering, Erosion, Deposition, and Lithification: Or How to Make a Sedimentary Rock/Reservoir Rock Weathering: Mechanical and/or chemical breakdown of rock material that creates sediments at or near the surface of the earth. Sediment: Fragmental or precipitated material transported and deposited by gravity, water, wind, ice or precipitation Mechanical vs. Chemical weathering: Mechanical:
1) 2) 3) 4)
Frost wedging Unloading Biological activity: roots, burrows Thermal expansion
Chemical:
1) Oxidation 2) Carbonation: carbon dioxide reacts with water to form carbonic acid 3) Hydrolysis
Mechanical weathering at work: Unloading (or pressure release – above) and Root wedging (right).
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PETR 571 Week 2‐notes
Classifying Sediments: Three S’s: Shape, Size, and Sorting Shape: Angular vs. Rounded Angular: sediment displays sharp corners and edges (transported over a short distance) Rounded: sediment has rounded, smooth edges (transported over a long distance)
Size: Almost all reservoir rocks are composed of sandstone/granule-size grains.
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PETR 571 Week 2‐notes
Sorting: A measure of the uniformity of grain size distribution within a sediment sample. Poorly sorted: particles of different sizes together Well sorted: particles of same size together •
Shape and sorting of grains are largely the dominant factors affecting porosity.
Erosion: Transportation of weathered material.
Mechanisms of erosion: 1) 2) 3) 4) 5) 6)
Running water, rivers (Alluvial/Fluvial) Wind (Eolian) Wave currents Gravity Ground water Glaciers
EOLIAN
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PETR 571 Week 2‐notes
Saltating sand grains.
Sand dunes.
Cross-bedding; Also seen in delta deposits (i.e., alluvial/ fluvial—common reservoir depositional environment).
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PETR 571 Week 2‐notes
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Different rocks are susceptible to weathering and erosion to different degrees.
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Sandstones are typically more resistant to weathering and erosion than shales.
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“Cropping-out” of beds can indicate the orientation of bedding and hence the subsurface geology.
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Deposition: Laying down of sediments at final erosional destination. •
The type of location at which sediments are deposited is referred to as the depositional environment.
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Depositional environments can be Continental, Coastal, or Marine
1) Continental: deserts, lakes, stream beds, swamps, caves 2) Coastal: deltas, sand bars, lagoons, estuaries 3) Marine: slope, ocean bottom (abyssal) •
Depositional environment strongly influences the likelihood of oil origination and accumulation
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Sediments of a common source or depositional event are grouped into strata
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Graded bed: strata whose sediments fine upward (i.e., grains gets smaller as approach top of bed)
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The stratigraphic column is a sequence of strata revealing depositional trends through time; correlating strata is widely used by petroleum geologists in the exploration of hydrocarbons
Graided bedding (right) with characteristic fining upward of grains.
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Lithification: The process of transforming loose, unconsolidated sediments into a rock. Can be accomplished through compaction, cementation, and crystallization.
Compaction: Weight of overlying sediments packs deeper grains together Cementation: Sediment grains are “cemented” together from the precipitation of mineral solute in pore space. Crystallization: “Cement” precipitates crystallize into minerals forming a “matrix” Matrix:
Fine-grained material occupying intergranular sedimentary pore space between coarser grains.
Sedimentary rock: Rocks formed from existing sediments through lithification.
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PETR 571 Week 2‐notes Glossary:
Abrasion: Mechanical wearing, grinding, or scraping, by impact and friction, of rock surfaces or grains by gravity, water, ice or wind. Alluvium: Comparatively geologically recent, unconsolidated, poorly sorted, detrital gravel, sand, silt and clay deposited by often ephemeral, rapidly moving water under flood or flash-flood conditions: stream, flood-plain, delta and alluvial fan deposits. Angular (grain): A grain form with sharp edges, irregular shape, and no rounding. Anhydrite: An evaporate mineral of calcium sulfate. Arenite: Consolidated, clastic rock of sand sized particles: arkose, sandstone, etc. Argillaceous: Shaly, or containing clayey constituents. Arkose: Coarse-grained, feldspathic, variably sorted sanstone containing angular grains, representing rapid deposition and limited grain transport. Basin: A low area with no exterior drainage. Often an area of sedimentary deposition: lake basin; marine basin. Bed: A stratum or layer of rock. Bedding: Layers of stratified rock. Bedding plane: A surface separating stratified rocks. Bed rock: Solid rock beneath soil or unconsolidated surficial material. Boundstone: Sedimentary carbonate rock the original components of which were bound together in place during deposition: most algal bank and reed deposits. Braided stream: A multiple channel stream divided because the alluvial material to be carried exceeds the capacity of the water to carry it. Calcareous: Rock or other material containing up to 50 percent calcium carbonate. Carbonate: Rock-forming minerals containing the carbonate ion which include calcite and dolomite. Carbonate platform: A substantial limestone or dolomite substrate upon which a reef might be built. Cementation: Precipitation of mineral material into intergranular or intercrystalline pore space. Chalk: Fine-textured marine limestone formed by shallow water accumulation of calcareous remains of floating micro-organisms and algae. Channel: A place through which a current can flow such as between two sand bars. Chemical weathering: Weathering by chemical change of mineral constituents in rocks. Clast: A grain or fragment. Clastic (rock): A rock composed of clasts.
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PETR 571 Week 2‐notes Compaction: Sediment volume decrease by increase in overburden pressure. Continental shelf: The area between the shore and the top of the continental slope. Continental slope: The inclined area between the continental shelf and the deep ocean: it averages about six degrees from the horizontal. Delta: Flat, commonly triangular, alluvial deposit occurring at the mouth of a river at its entry to a quiet body of water, i.e., lake or ocean. Deposition: The laying down or emplacement of material, especially sedimentary, as stratified or unstratified accumulations. Detritus: An accumulation of mechanically derived rock and mineral fragments including gravel, sand and silt. Diagenesis: The process of converting sediment to rock. Drift: All glacially originated, transported and deposited material. Eolian: Wind-blown or wind-related. Erosion: Removal of rock material to another place by one or several transportation agencies. Evaporite: A rock or mineral deposited by precipitation during evaporation. Fluvial: Pertaining to rivers and streams. Friable: Easily pulverized or crumbly rock or mineral material. Frost wedging: Dislocation, prying and mechanical breakdown of fractured rock by expansion of ice in the fractures. Glacial erosion: Glacial removal and transportation of rock material. Lacustrine: Pertaining to lakes and lake environments. Lithification: Solidification of sediment to rock: induration, diagenesis. Mechanical weathering: Physical break down of rock material. Oolite: A sedimentary rock comprising concentrically precipitated calcium carbonate ooliths approximately 1 mm in diameter. Sediment: Fragmental or precipitated material transported and deposited by gravity, water, wind, ice or precipitation. Silica: Silicon dioxide, quartz. Siliceous: A rock containing abundant free silica. Sorting: The degree of constancy of grain size in a clastic rock. Well-sorted rocks comprise grains of the same size. The dynamic process of achieving sorting of grains. Turbidite: A turbidity current-deposited, graded clastic sequence.
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PETR 571 Week 2‐notes Turbidity current: A gravity-motivated current containing velocity suspended sediment which deposits graded sediments as its velocity decreases. Weathering: Mechanical and/or chemical breakdown of rock material.
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