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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:
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).
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
distance)
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
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
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
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.
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”
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
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.
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.
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