Kerogen is a solid organic matter in sedimentary rocks. Consisting of an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter by 10,000 fold. It is insoluble in normal organic solvents and it does not have a specific chemical formula. Upon heating, kerogen converts in part to liquid and gaseous hydrocarbons. Petroleum and natural gas form from kerogen. Based on its origin, kerogen may be classified as algal, mixed terrestrial and marine.
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. As kerogen is a mixture of organic material, rather than a specific chemical, it cannot be given a chemical formula. Its chemical composition can vary distinctively from sample to sample. For example, 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.
Kerogen is insoluble in normal organic solvents because of the high molecular weight (upwards of 1,000 daltons or 1000 Da; 1 Da = 1 atomic mass unit) of its component compounds. The soluble portion is known as bitumen. When heated to the right temperatures in the Earth's crust, (oil window c. 50–150 °C, gas window c. 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 a warmer temperature to release their hydrocarbons may form oil shale deposits.
Kerogen arises from the degradation of living matter, such as diatoms, planktons, spores and pollens. In this break-down process, large biopolymers from proteins and carbohydrates dismantle partially or completely. (This break-down process can be viewed as the reverse of photosynthesis). These dismantled components are units that can then polycondense to form polymers. This polymerization usually happens alongside the formation of a mineral component (geopolymer) resulting in a sedimentary rock like kerogen shale.
The formation of polymers in this way accounts for the large molecular weights and diverse chemical compositions associated with kerogen. The smallest units are the fulvic acids, the medium units are the humic, and the largest units are the humins. When organic matter is contemporaneously deposited with geologic material, subsequent sedimentation and progressive burial or overburden provide significant pressure and a temperature gradient. When these humic precursors are subjected to sufficient geothermal pressures for sufficient geologic time, they begin to undergo certain specific 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.
Kerogen breaks down in the subsurface to generate oil and gas, which form the source of hydrocarbons in conventional reservoirs. In unconventional resources, many of which are referred to as shale, the produced hydrocarbons have not been expelled from the source rock, but instead are stored and transported within the shale. Most kerogens of relevance to the oil and gas industry are marine (type II). Much of the porosity in shale is hosted within kerogen, and the recent development of economic shale resources has led to increased research into the composition of kerogen. Studies using NMR spectroscopy have found that carbon in kerogen can range from almost entirely aliphatic (sp3 hybridized) to almost entirely aromatic (sp2 hybridized). with kerogens of higher type and/or higher thermal maturity typically having higher abundance of aromatic carbon.
Type I: Sapropelic
Type 1 oil shales yield larger amount of volatile or extractable compounds than other types upon pyrolysis. Hence, from the theoretical view, Type 1 kerogen oil shales provide the highest yield of oil and are the most promising deposits in terms of conventional oil retorting.
- 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: Planktonic
Type II kerogen is common in many oil shale deposits. It is based on marine organic materials, which are formed in reducing environments. Sulfur is found in substantial amounts in the associated bitumen and generally higher than the sulfur content of Type I or III. Although pyrolysis of Type II kerogen yields less oil than Type I, the amount acquired is still sufficient to consider Type II bearing rocks as potential oil sources
- Plankton (marine)
- 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: Sulfurous
Similar to Type II but high in sulfur.
Type III: Humic
- Land plants (coastal)
- 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 III kerogen involving rocks are found to be the least productive upon pyrolysis and probably the least favorable deposits for oil generation
Type IV: Residue
Hydrogen: carbon ratio < 0.5
Carbonaceous chondrite meteorites contain kerogen-like components. Such material is thought to have formed the terrestrial planets. Kerogen materials have been detected also in interstellar clouds and dust around stars.
The Curiosity rover has detected organic deposits similar to kerogen in mudstone samples in Gale Crater on Mars using a revised drilling technique. The presence of benzene and propane also indicates the possible presence of kerogen-like materials, from which hydrocarbons are derived.
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- Oil shale geology
- Petroleum geology – The study of the origin, occurrence, movement, accumulation, and exploration of hydrocarbon fuels
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