Kerogen

Kerogens are chemical compounds, often found to be formed by the low-grade metamorphism (i.e. diagenesis) of organic molecules derived from decaying plant and animal matter. These are long-chain polymers which do not dissolve in several specific solvents.

Kerogens are the precursors to hydrocarbons (fossil fuels).

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

• Hydrogen:Carbon ratio > 1.25
• Oxygen:Carbon ratio < 0.15
• Tend to produce oil.

Type II

• Hydrogen:Carbon ratio < 1.25
• Oxygen:Carbon ratio 0.03 to 0.18
• Tend to produce a mix of gas and oil.

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.
• Tend to produce gas.

Type IV (residue)

• Hydrogen:Carbon < 0.5

Origin 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: petroleum geology.