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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:
Frost wedging
Biological activity: roots, burrows
Thermal expansion
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).
1 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
Size: Almost all reservoir rocks are composed of sandstone/granule-size grains.
2 PETR 571 Week 2‐notes Sorting: A measure of the uniformity of grain size distribution within a sediment
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:
Running water, rivers (Alluvial/Fluvial)
Wind (Eolian)
Wave currents
Ground water
3 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).
4 PETR 571 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.
5 PETR 571 Week 2‐notes 6 PETR 571 Week 2‐notes 7 PETR 571 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
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.
8 PETR 571 Week 2‐notes 9 PETR 571 Week 2‐notes 10 PETR 571 Week 2‐notes 11 PETR 571 Week 2‐notes 12 PETR 571 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”
Fine-grained material occupying intergranular sedimentary pore space
between coarser grains.
Sedimentary rock: Rocks formed from existing sediments through lithification.
13 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.
14 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
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
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
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.
15 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.