kerogen

Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks.^[1] 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°-120°C, gas window ca.120°-150°C) 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 a sufficient 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.^[2][3]

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 C 215 : H 330 : O 12 : N 5 : S 1.^[2]

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.^[4]

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: 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) <ref: Krause FF, 2009>
• 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.

Most of the biomass that eventually becomes petroleum is contributed by the bacteria and protists that decompose the primary matter, not the primary matter itself. However, the lignin in this kerogen decomposes to form phenolic compounds that are toxic to bacteria and protists. Without this extra input, it will only become methane and/or coal.

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.^[5] Such material is believed to have formed the terrestrial planets.
• Kerogen materials have been detected in interstellar clouds and dust around stars.^[6]

See also

• Asphaltene
• Oil shale geology
• Petroleum geology
• Tholin

References

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External links

• European Association of Organic Geochemists
• Organic Geochemistry (journal)