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Kerogen
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Kerogens are the precursors to Hydrocarbon s ( Fossil Fuel s), and are also the material that forms Oil Shales .




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 and Amorphous Organic Matter (AMO)

  • Hydrogen:Carbon ratio > 1.25

  • Oxygen:Carbon ratio < 0.15

  • Shows great tendency to readily produce liquid hydrocarbons, but its occurrence is extremely limited and does not warrant the attention given to it.

  • 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 pollen and spores

  • Cutinite: formed from terrestrial plant cuticle

  • Resinite: terrestrial plant resins, 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

  • 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 Lipid s or waxy matter. It forms from Cellulose , the carbohydrate polymer that forms the rigid structure of terrestrial plants, Lignin , another carbohydrate polymer (polysaccharide) that binds the strings of cellulose together, and Terpene s and Phenol ic 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 Hydrocarbon s. 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




See also: Petroleum Geology .