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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) Frost wedging
- Unloading
- Biological activity: roots, burrows
- Thermal expansion
Chemical: 1) Oxidation
- Carbonation: carbon dioxide reacts with water to form carbonic acid
- Hydrolysis
Mechanical weathering at work: Unloading (or pressure release – above) and Root wedging (right).
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.
Week 2 ‐notes
Saltating sand grains.
Sand dunes.
Cross-bedding; Also seen in delta deposits (i.e., alluvial/ fluvial—common reservoir depositional environment).
Week 2 ‐notes
- Different rocks are susceptible to weathering and erosion to different degrees.
- Sandstones are typically more resistant to weathering and erosion than shales.
- “Cropping-out” of beds can indicate the orientation of bedding and hence the subsurface geology.
Week 2 ‐notes
Week 2 ‐notes
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.
- Depositional environments can be Continental, Coastal, or Marine
- Continental: deserts, lakes, stream beds, swamps, caves
- Coastal: deltas, sand bars, lagoons, estuaries
- Marine: slope, ocean bottom (abyssal)
- Depositional environment strongly influences the likelihood of oil origination and accumulation
- Sediments of a common source or depositional event are grouped into strata
- Graded bed : strata whose sediments fine upward (i.e., grains gets smaller as approach top of bed)
- 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.
Week 2 ‐notes
Week 2 ‐notes
Week 2 ‐notes
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.
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.
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.
