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 quickly 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, 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. These dismantled components can come together to form new polymers 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, the medium geopolymers are the humic, and the largest geopolymers are the humins. When organic matter is contemporaneously deposited with geologic material, subsequent Sedimentation and progressive burial or overburden provide significant pressure and temperature gradient. 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 which are associated 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, characteristically 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.

Types

Labile kerogen breaks down to form heavy hydrocarbons (i.e. oils), refractory kerogen breaks down to form light hydrocarbons (i.e. gases), and inert kerogen forms graphite.

A Van Krevelen diagram is one example of classifying kerogens, where they tend to form groups when the ratios of hydrogen to carbon and oxygen to carbon are compared.

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 lacustrine 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:
  • Sporinite: 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)
• Has very low hydrogen because of the extensive ring and aromatic systems

Kerogen Type III is formed from terrestrial plant matter that is lacking in lipids or waxy matter. It forms from cellulose, the carbohydrate polymer that forms the rigid structure of terrestrial plants, lignin, a non-carbohydrate polymer formed from phenyl-propane units that binds the strings of cellulose together, and terpenes and phenolic compounds in the plant.

Type IV (residue)

• Hydrogen:carbon < 0.5

Type IV kerogen contains mostly decomposed organic matter in the form of polycyclic aromatic hydrocarbons. They have no potential to produce hydrocarbons.

Origin of material

Terrestrial material

The type of material is difficult to determine but several apparent patterns have been noticed.

• Ocean or lake material often meet kerogen type III or IV classifications.
• Ocean or lake material deposited under anoxic conditions often form kerogens of type I or II.
• Most higher land plants produce kerogens of type III or IV.
• Some coal contains type II kerogen.

Extraterrestrial material

• Carbonaceous chondrite meteorites contain kerogen-like components. 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

External links

• European Association of Organic Geochemists
• Organic Geochemistry (journal)ar:كيروجين