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Organic petrology is the comprehensive investigation of organic materials occurring in sedimentary rocks; in particular, coal and petroleum source rocks. Organic petrology also includes study of the utilization products of coal and petroleum source rocks, for instance, coke, fly ash, and liquefaction residue. Studies of the origin, occurrence, structure, and history of sedimentary organic matter typically are pursued through techniques of optical microscopy. Complementary geochemical methods of investigation include pyrolysis, gas and liquid chromatography, solvent extraction, and mass spectroscopy.
Organic petrology research is used to understand and predict the behavior of coal in utilization and/or thermal maturity of petroleum source rocks, and to understand and predict hydrocarbon generation. Other important themes of organic petrology research include the impact of coal composition on coalbed gas generation and storage. Research in the USGS Organic Petrology Laboratory at the National Center in Reston includes all of these themes.
Paul HackleyProject Chief
The individual organic components of coal are termed “macerals,” similar to “minerals” in rock. Macerals are divided into three broad groups: vitrinite, inertinite, and liptinite. Vitrinites are the coalified remains of humic plant substances, primarily lignin and cellulose. In most cases, inertinites consist of the same original plant material as vitrinite but have been altered by charring or oxidation prior to coalification... [+]
In most cases, inertinites consist of the same original plant material as vitrinite but have been altered by charring or oxidation prior to coalification. Inertinites can also include fungal bodies. Liptinites are the remains of hydrogen-rich plant materials such as cutins, resins, fats, waxes, andsporopollenin (the outer cell walls of spores and pollen). The maceral content of coal is determined with a reflected light microscope (light is reflected from the sample towards the analyst) at magnifications of about 500x, using tungsten filament and gas arc light sources. Photomicrographs of USGS coal samples are contained in the organic petrology photomicrograph atlas.
Stanton, R.W., Warwick, P.D., and Swanson, S.M., 2005, Tar yields from low-temperature carbonization of coal facies from the Powder River Basin, Wyoming, USA: International Journal of Coal Geology, v. 63, p. 13-26.
Hackley, P.C., Warwick, P.D., and Gonzáles, E., 2005, Petrology, mineralogy, and geochemistry of mined coals, western Venezuela: International Journal of Coal Geology, v. 63, p. 68-97.
Measurement of the reflectance of the maceral vitrinite is a commonly performed analysis that is used to establish the rank of coals or the thermal maturity of petroleum source rocks. A well-established correlation exists between vitrinite reflectance and rank due to increases in the refractive and adsorption indices of vitrinite with increasing aromatization. Coal or source rocks samples are prepared in polished briquettes... [+]
which are then compared to the reflectance of calibrated glass standards on a reflected light microscope. The USGS is currently involved in maturation studies of coals and rock in support of coalbed methane exploration in Louisiana, Arkansas, and Texas.
Hackley, P.C., Guevara, E. H., Hentz, T.F., and Hook, R.W., 2009, Thermal maturity and organic composition of Pennsylvanian coals and carbonaceous shales, north-central Texas: implications for coalbed gas potential: International Journal of Coal Geology, v. 77, p. 294-309, doi:10.1016/j.coal.2008.05.006.
The maceral composition of coals may impact the ability of coal to generate and/or store natural gases, primarily methane. The USGS is actively pursuing studies to elucidate the role of maceral content on gas generation and storage, primarily in the Gulf Coast region of the United States. Coal samples from coalbed methane test wells are desorbed to measure actual stored gas content, microscopically analyzed for organic... [+]
content, and subjected to adsorption isotherm analyses to measure the coal’s ability to store gas. These comparative studies allow predictions of the role of maceral content on gas storage and may help in understanding gas generation. For more information pertaining to coalbed methane, please visit the "Oil and Gas" section of this website.
Hackley, P.C., Warwick, P.D., and Breland, F.C., Jr., 2007, Organic petrology and coalbed gas content, Wilcox Group, northern Louisiana: International Journal of Coal Geology, v. 71, p. 54-71, doi:10.1016/j.coal.2006.05.009.
Warwick, P.D., Breland, F.C., Jr., and Hackley, P.C., 2008, Biogenic origin of coalbed gas in the northern Gulf of Mexico Coastal Plain, USA: International Journal of Coal Geology, v. 76, p. 119-137, doi:10.1016/j.coal.2008.05.009.
The types and amounts of organic material preserved in coal beds are indicators of the environmental processes that operated in the original peat mires. In conjunction with other types of analytical and geologic data, the organic content of coals can be used to infer moisture stress (frequency and degree of fluctuations in the water table), depositional environment (acidity, mechanical stress), types and relative amounts of original plant... [+]
vegetation, tectonic environment (subsidence), and climateThe USGS Organic Petrology Laboratory is involved in a number of national and international projects which characterize paleoenvironments through organic petrology.
Hackley, P.C., and Martínez, M., 2007, Organic petrology of Paleocene Marcelina Formation coals, Paso Diablo mine, western Venezuela: Tectonic controls on coal type: International Journal of Coal Geology, v. 71, p. 505-526, doi:10.1016/j.coal.2006.05.002.
Hackley, P.C., SanFilipo, J.R., Azizi, G.P., Davis, P.A., and Starratt, S.W., 2010, Organic petrology of subbituminous carbonaceous shale samples from Chalâw, Kabul Province, Afghanistan: speculations on paleoenvironment and energy resource potential: International Journal of Coal Geology, v. 81, p. 269-280, doi:10.1016/j.coal.2009.12.007.
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Database of the United States Coal Pellet Collection of the U.S. Geological Survey Organic Petrology Laboratory
Energy Resources Program - Geochemistry Research
Energy Resource Program - Geochemistry Laboratories
American Association of Petroleum Geologist (AAPG)
American Society for Testing and Materials (ASTM)
Canadian Society for Coal Science & Organic Petrology (CSCOP)
Geological Society of America (GSA) - Coal Geology
International Committee for Coal & Organic Petrology (ICCP)
The Society for Organic Petrology (TSOP)
Society for Sedimentary Geology (SEPM)
Page Last Modified: Friday, June 19, 2015