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.[1] 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 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.

The name "kerogen" was introduced by the Scottish organic chemist Alexander Crum Brown in 1906.[2][3][4][5]

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.[6] 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[7]). 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.[8]

Composition

As kerogen is a mixture of organic material, rather than a specific chemical, it cannot be given a chemical formula. Indeed, its chemical composition can vary distinctively from sample to sample. 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.[9]

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.[10]

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 [11]

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

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

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

Type IV kerogen contains mostly decomposed organic matter in the form of polycyclic aromatic hydrocarbons. They have no potential to produce hydrocarbons.[13]

Origin of material

Terrestrial

The type of material is difficult to determine, but several apparent patterns have been noticed.

Extra-terrestrial

See also

References

  1. ↑ Oilfield Glossary
  2. ↑ Oxford English Dictionary 3rd Ed. (2003)
  3. ↑ Teh Fu Yen; Chilingar, George V. (1976). Oil Shale. Amsterdam: Elsevier. p. 27. ISBN 978-0-444-41408-3. Retrieved 31 May 2009.
  4. ↑ Hutton, Adrian C.; Bharati, Sunil; Robl, Thomas (1994). "Chemical and Petrographic Classification of Kerogen/Macerals". Energy Fuels. Elsevier Science. 8 (6): 1478–1488. doi:10.1021/ef00048a038.
  5. ↑ 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."
  6. ↑ 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).
  7. ↑ Tucker M.E. (1988) Sedimentary Petrology, An Introduction, Blackwell, London. p197. ISBN 0-632-00074-0
  8. ↑ 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).
  9. ↑ Teh Fu Yen; Chilingar, George V. (1976). Oil Shale. Amsterdam: Elsevier. ISBN 978-0-444-41408-3.
  10. ↑ Example of a Van Krevelen diagram
  11. ↑ Tissot B. P., Welte D. H., “Petroleumformation and occurrence”, Springer Verlag Germany, 1984.
  12. ↑ Krause FF, 2009
  13. ↑ Weber G., Green J., ‘‘Guide to oil shale’’. NationalConference of State Legislatures. Washington D.C. USA.p. 21, 1981.
  14. ↑ Nakamura, T. (2005) "Post-hydration thermal metamorphism of carbonaceous chondrites", Journal of Mineralogical and Petrological Sciences, volume 100, page 268, (PDF) Retrieved 1 September 2007
  15. ↑ Papoular, R. (2001) "The use of kerogen data in understanding the properties and evolution of interstellar carbonaceous dust", Astronomy and Astrophysics, volume 378, pages 597-607, (PDF) Retrieved 1 September 2007

External links

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