Kerogen

Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight (upwards of 1,000 Daltons) of its component compounds. The soluble portion is known as bitumen. When heated to the right temperatures in the Earth's crust, (oil window ca. 60°-160°C, gas window ca.150°->200°C, both depending on how fast the source rock is heated) some types of kerogen release crude oil or natural gas, collectively known as hydrocarbons (fossil fuels). When such kerogens are present in high concentration in rocks such as shale they form possible source rocks. Shales rich in kerogens that have not been heated to warm temperature to release their hydrocarbons may form oil shale deposits.

The name "kerogen" was introduced by the Scottish organic chemist Alexander Crum Brown in 1912.

Formation of kerogen

At the demise of living matter, such as diatoms, planktons, spores and pollens for example, the organic matter begins to undergo decomposition or degradation. In this break-down process, (which is basically the reverse of photosynthesis ), large biopolymers from proteins and carbohydrates begin to partially or completely dismantle. Akin to survival principles, the dismantled components come together to form or constitute new unique polymers appropriately referred to as geopolymers. Geopolymers are the precursors of kerogen.

The formation of geopolymers in this way, accounts for the large molecular weights and diverse chemical compositions associated with kerogen. The smallest geopolymers are the fulvic acids, medium geopolymers are the humic and the largest geopolymers are the humins. Sedimentation and progressive burial or overburden provides significant pressure and temperature gradient. So when geopolymers are subjected to sufficient geothermal pressures for sufficient geologic time, they begin to undergo certain peculiar changes to become kerogen. Such changes are indicative of the maturity stage of a particular kerogen. These changes include loss of hydrogen, oxygen, nitrogen, and sulfur, which leads to loss of other functional groups that further promote isomerization and aromatization with increasing depth or burial. Aromatization then allows for neat molecular stacking in sheets, which in turn increases molecular density and vitrinite reflectance properties, as well as changes in spore coloration, from yellow to orange to brown to black with increasing depth.

Composition

As kerogen is a mixture of organic material, rather than a specific chemical; it cannot be given a chemical formula. Indeed its chemical composition can vary distinctively from sample to sample. Kerogen from the Green River Formation oil shale deposit of western North America contains elements in the proportions carbon 215 : hydrogen 330 : oxygen 12 : nitrogen 5 : sulfur 1.

Type I

containing alginite, amorphous organic matter, cyanobacteria, freshwater algae, and land plant resins

Hydrogen:carbon ratio > 1.25

Oxygen:carbon ratio < 0.15

Shows great tendency to readily produce liquid hydrocarbons.

It derives principally from algae and forms only in anoxic lakes and several other unusual marine environments

Has few cyclic or aromatic structures

Formed mainly from proteins and lipids

Type II

Hydrogen:carbon ratio < 1.25

Oxygen:carbon ratio 0.03 to 0.18

Tend to produce a mix of gas and oil.

Several types: exinite, cutinite, resinite, and liptinite

* Exinite: formed from the casings of pollen and spores

* Cutinite: formed from terrestrial plant cuticle

* Resinite: formed from terrestrial plant resins and animal decomposition resins

* Liptinite: formed from terrestrial plant lipids (hydrophobic molecules that are soluble in organic solvents) and marine algae

They all have great tendencies to produce petroleum and are all formed from lipids deposited under reducing conditions.

Type II–sulfur

Similar to Type II but high in sulfur.

Type III

Hydrogen:carbon ratio < 1

Oxygen:carbon ratio 0.03 to 0.3

Material is thick, resembling wood or coal.

Tends to produce coal and gas (Recent research has shown that type III kerogens can actually produce oil under extreme conditions) Such material is believed to have formed the terrestrial planets. Kerogen materials have been detected in interstellar clouds and dust around stars.

See also

Asphaltene

Oil shale geology

Petroleum geology

Tholin

References

External links

European Association of Organic Geochemists

Organic Geochemistry (journal)

Category:Soil science Category:Petroleum products Category:Oil shale