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kerogen
Kerogen (Greek κηρός "wax" and -gen, γένεση "birth") is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks.〔(Oilfield Glossary )〕 It is insoluble in normal organic solvents because of the high molecular weight (upwards of 1,000 daltons or 1000 Da; 1Da= 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'' ca. 50–150 °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 a warmer temperature to release their hydrocarbons may form oil shale deposits.
The name "kerogen" was introduced by the Scottish organic chemist Alexander Crum Brown in 1906.〔''Oxford English Dictionary'' 3rd Ed. (2003)〕〔D. R. Steuart in H. M. Cadell et al. ''Oil-Shales of Lothians'' iii. 142 (1906) "We are indebted to Professor Crum Brown, F.R.S., for suggesting the term Kerogen to express the carbonaceous matter in shale that gives rise to crude oil in distillation."〕
==Formation of kerogen==
With the demise of living matter, such as diatoms, planktons, spores and pollens, the organic matter begins to undergo decomposition or degradation.〔Kudzawu-D'Pherdd, R., 2010. "The Genesis of Kerogen, a write up in Petroleum Geochemistry" - (EASC 616), Department of Earth Science, University of Ghana-Legon, ''(unpublished)''.〕 In this ''break-down'' process, large biopolymers from proteins and carbohydrates begin to dismantle either partially or completely. (According to Maurice Tucker (1988), this ''break-down'' process is basically the reverse of photosynthesis〔Tucker M.E. (1988) ''Sedimentary Petrology, An Introduction'', Blackwell, London. p197. ISBN 0-632-00074-0〕). 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.〔Kudzawu-D'Pherdd, R., 2010. "The Genesis of Kerogen, a write up in Petroleum Geochemistry" - (EASC 616), Department of Earth Science, University of Ghana-Legon, (unpublished).〕
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