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The Weathering Erosion, Deposition and the Lithification | PETR 571, Study notes of Engineering

Material Type: Notes; Class: Unconventional Oil & Gas; Subject: Petroleum Engineering; University: New Mexico Institute of Mining and Technology; Term: Unknown 1989;

Typology: Study notes

Pre 2010

Uploaded on 08/08/2009

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PETR571
Week2notes
1
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
2) Unloading
3) Biological activity: roots, burrows
4) 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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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

  1. Unloading
  2. Biological activity: roots, burrows
  3. Thermal expansion

Chemical: 1) Oxidation

  1. Carbonation: carbon dioxide reacts with water to form carbonic acid
  2. 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
  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
  • 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.