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Diagenesis
Diagenesis ( lithification) includes the full range of
alterations sediments undergo after deposition, at relatively low
temperatures and pressures (gradational to metamorphism)
Lithification may occur simultaneously with deposition (in
several carbonates, evaporites, and volcanoclastics)
Physical and chemical diagenetic processes constitute compaction
and cementation, respectively
Diagenesis leads to a reduction of porosity and permeability
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Diagenesis
Compaction
Compaction is the result of overburden pressure during sediment
burial, resulting in a decrease of volume and an increase of
density Compaction is extremely important in organics and muds, but
less important
in sands, gravels, and reefal carbonates Compaction is
accompanied by the expulsion of groundwater and a reduction
of porosity Differential compaction is important when sediments
exhibit a high
spatial variability
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Diagenesis
Compaction
Pressure dissolution results in increasingly interlocking
grains, and significantly contributes to lithification
In limestones, pressure dissolution usually occurs at specific
horizons, that may or may not correspond to depositional bedding
planes
Stylolites are irregular pressure dissolution surfaces with
higher proportions of residual material and represent more extreme
forms of this process
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Diagenesis
Cementation
Dissolution commonly occurs without high pressures, and
subsequent precipitation results in the formation of cement
(authigenic minerals) Calcium carbonate (sparry or micritic) Silica
(commonly microquartz) Clay minerals
Cementation reduces both the porosity and the permeability
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Diagenesis
Cementation
Nodules (irregular) and concretions (rounded) are larger
cemented bodies (e.g., silica, calcite, siderite, pyrite) Chert
(flint) is the most widely known type of silica nodules,
especially
common in limestones
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Diagenesis
Dolomitization
Dolomites are mostly formed diagenetically, involving the
replacement of calcite or aragonite by dolomite (CaMg(CO3)2)
Four main models of dolomitization can be distinguished:
Evaporite brine residue/seepage reflux model
Meteoric-marine/groundwater mixing model (obsolete) Burial
compaction/formation water model Sea water/convection model
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Diagenesis
Coal formation
Coal formation is primarily the result of compaction and
geothermal heating
A very high proportion of compaction occurs during the
peat-accumulation stage Peat --> lignite --> bituminous coal
--> anthracite Relative increase of carbon over hydrogen and
oxygen (gradual expulsion of
H2O, CO2, and CH4) Methane (CH4) is an important byproduct of
coal formation
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Diagenesis
Hydrocarbon formation
Diagenetic breakdown of planktonic algae (maturation) leads to
the formation of kerogen (long-chain hydrocarbons)
Liquid hydrocarbons (shorter-chain hydrocarbons) are generally
formed at temperatures of 70-100 C (oil window at 2-3 km depth)
Methane is released at temperatures over 150 C
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