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

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

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

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

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

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

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

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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