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Kerogen is solid, insoluble organic matter in . It consists of a variety of organic materials, including dead plants, algae, and other microorganisms, that have been compressed and heated by geological processes. All the kerogen on earth is estimated to contain 1016 tons of carbon. This makes it the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold.
The type of kerogen present in a particular rock formation depends on the type of organic material that was originally present. Kerogen can be classified by these origins: lacustrine (e.g., Algae), Marine ecosystem (e.g., planktonic), and terrestrial (e.g., pollen and ). The type of kerogen depends also on the degree of heat and pressure it has been subjected to, and the length of time the geological processes ran. The result is that a complex mixture of organic compounds resides in sedimentary rocks, serving as the precursor for the formation of such as oil and gas. In short, kerogen amounts to fossilized organic matter that has been buried and subjected to high temperatures and pressures over millions of years, resulting in various chemical reactions and transformations.
Kerogen is insoluble in normal 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. The name "kerogen" was introduced by the Scottish organic chemist Alexander Crum Brown in 1906, Oxford English Dictionary 3rd Ed. (2003)Steuart, D.R., in Cadell, H.M. 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." derived from the Greek words for wax and origin (Greek: κηρός "wax" and -gen, γένεσις "origin").
The increased production of shale oil has motivated a revival of research into the composition, structure, and properties of kerogen. Many studies have documented dramatic and systematic changes in kerogen composition across the range of thermal maturity relevant to the oil and gas industry. Analyses of kerogen are generally performed on samples prepared by acid demineralization with critical point drying, which isolates kerogen from the rock matrix without altering its chemical composition or microstructure.
When kerogen is contemporaneously deposited with geologic material, subsequent sedimentation and progressive burial or overburden provide elevated pressure and temperature owing to lithostatic and geothermal gradients in Earth's crust. Resulting changes in the burial temperatures and pressures lead to further changes in kerogen composition including loss of hydrogen, oxygen, nitrogen, sulfur, and their associated functional groups, and subsequent isomerization and aromatization Such changes are indicative of the thermal maturity state of kerogen. Aromatization allows for molecular stacking in sheets, which in turn drives changes in physical characteristics of kerogen, such as increasing molecular density, vitrinite reflectance, and spore coloration (yellow to orange to brown to black with increasing depth/thermal maturity).
During the process of thermal maturation, kerogen breaks down in high-temperature pyrolysis reactions to form lower-molecular-weight products including bitumen, oil, and gas. The extent of thermal maturation controls the nature of the product, with lower thermal maturities yielding mainly bitumen/oil and higher thermal maturities yielding gas. These generated species are partially expelled from the kerogen-rich source rock and in some cases can charge into a reservoir rock. Kerogen takes on additional importance in unconventional resources, particularly shale. In these formations, oil and gas are produced directly from the kerogen-rich source rock (i.e. the source rock is also the reservoir rock). Much of the porosity in these shales is found to be hosted within the kerogen, rather than between mineral grains as occurs in conventional reservoir rocks. Thus, kerogen controls much of the storage and transport of oil and gas in shale.
Another possible method of formation is that vanabin-containing organisms cleave the core out of chlorin-based compounds such as the magnesium in chlorophyll and replace it with their vanadium center in order to attach and harvest energy via light-harvesting complexes. It is theorized that the bacteria contained in worm castings, Rhodopseudomonas palustris, do this during its photoautotrophism mode of metabolism. Over time colonies of light harvesting bacteria solidify, forming kerogen .
Kerogen is insoluble in normal organic solvents in part because of the high molecular mass 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 Celsius scale, 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 (). When such kerogens are present in high concentration in rocks such as organic-rich mudrocks shale, they form possible . Shales that are rich in kerogen but have not been heated to required temperature to generate hydrocarbons instead may form oil shale deposits.
The chemical composition of kerogen has been analyzed by several forms of solid state spectroscopy. These experiments typically measure the speciations (bonding environments) of different types of atoms in kerogen. One technique is 13C NMR spectroscopy, which measures carbon speciation. NMR experiments have found that carbon in kerogen can range from almost entirely aliphatic ( sp3 hybridized) to almost entirely aromatic ( sp2 hybridized), with kerogens of higher thermal maturity typically having higher abundance of aromatic carbon. Another technique is Raman spectroscopy. Raman scattering is characteristic of, and can be used to identify, specific vibrational modes and symmetries of molecular bonds. The first-order Raman spectra of kerogen comprises two principal peaks; a so-called G band ("graphitic") attributed to in-plane vibrational modes of well-ordered sp2 carbon and a so-called D band ("disordered") from symmetric vibrational modes of sp2 carbon associated with lattice defects and discontinuities. The relative spectral position (Raman shift) and intensity of these carbon species is shown to correlate to thermal maturity, with kerogens of higher thermal maturity having higher abundance of graphitic/ordered aromatic carbons. Complementary and consistent results have been obtained with infrared (IR) spectroscopy, which show that kerogen has higher fraction of aromatic carbon and shorter lengths of aliphatic chains at higher thermal maturities. These results can be explained by the preferential removal of aliphatic carbons by cracking reactions during pyrolysis, where the cracking typically occurs at weak C–C bonds beta to aromatic rings and results in the replacement of a long aliphatic chain with a methyl group. At higher maturities, when all labile aliphatic carbons have already been removed—in other words, when the kerogen has no remaining oil-generation potential—further increase in aromaticity can occur from the conversion of aliphatic bonds (such as alicyclic rings) to aromatic bonds.
IR spectroscopy is sensitive to carbon-oxygen bonds such as , , and , so the technique can also be used to investigate oxygen speciation. It is found that the oxygen content of kerogen decreases during thermal maturation (as has also been observed by elemental analysis), with relatively little observable change in oxygen speciation. Similarly, sulfur speciation can be investigated with X-ray absorption near edge structure (XANES) spectroscopy, which is sensitive to sulfur-containing functional groups such as , , and . Sulfur content in kerogen generally decreases with thermal maturity, and sulfur speciation includes a mix of sulfides and thiophenes at low thermal maturities and is further enriched in thiophenes at high maturities.
Overall, changes in kerogen composition with respect to heteroatom chemistry occur predominantly at low thermal maturities (bitumen and oil windows), while changes with respect to carbon chemistry occur predominantly at high thermal maturities (oil and gas windows).
In organic petrography, the different components of kerogen can be identified by microscopic inspection and are classified as . This classification was developed originally for coal (a sedimentary rock that is rich in organic matter of terrestrial origin) but is now applied to the study of other kerogen-rich sedimentary deposits.
The Van Krevelen diagram is one method of classifying kerogen by "types", where kerogens form distinct groups when the ratios of hydrogen to carbon and oxygen to carbon are compared. Example of a Van Krevelen diagram.
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.Archived at Ghostarchive and the
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Marakushev, S.A.; Belonogova, O.V. (2021), "An inorganic origin of the “oil-source” rocks carbon substance". Georesursy = Georesources. 23
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