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AAPG Bulletin


Generation and Expulsion of Petroleum and Gas from Almond Formation Coal, Greater Green River Basin, Wyoming1

Mario García-González2, Ronald C. Surdam3, and Milton L. Lee4


Petrographic and geochemical studies of coal from the Almond Formation in the Greater Green River basin demonstrate that the coal contains important volumes of stored liquid petroleum, as well as methane. Modeling indicates that at the basin center, most of the oil generated in the coal has been thermally cracked to gas, whereas at the basin flank the oil-to-gas reaction has barely proceeded.

Several new concepts are presented about the mechanism of petroleum generation in coal based on (1) natural maturation trends gleaned from examination of Almond coal samples from different burial depths and (2) similar maturation trends observed in hydrous pyrolysis experiments using immature Almond coal samples. These new concepts show that the oil in the coal was generated during the alteration of desmocollinite and liptinite macerals to exsudatinite (waxy oil) and inertinite solid residue; that the waxy oil was initially stored in porous structures and subsequently in vesicles as the coal matured under increasing temperature; that primary migration of the oil occurred as the generation of a sufficient volume of exsudatinite microfractured the vitrinite-
semifusinite vesicles, interconnecting vesicles, 

©Copyright 1996. The American Association of Petroleum Geologists. All rights reserved.

1Manuscript received August 1, 1995; revised manuscript received April 23, 1996; final acceptance July 15, 1996.

2Universidad Industrial de Santander, P. O. Box 678, Bucaramanga, Colombia.

3University of Wyoming, Laramie, Wyoming 82071.

4Brigham Young University, Provo, Utah 84602.

We wish to thank Donald McGowan for discussions that improved this paper, Francis P. Miknis (Western Research Institute, University of Wyoming) for the NMR analyses, Steven J. Butala (Department of Chemistry, Brigham Young University) for the gas chromatography/mass spectrometry analyses, and Steven Boese (Department of Geology and Geophysics, University of Wyoming) for his analytical help. We would also like to acknowledge David Copeland and Kathy Kirkaldie (Institute for Energy Research, University of Wyoming), who greatly improved the manuscript with many suggestions.

This research was supported under Gas Research Institute Contracts No. 5091-221-2146 and 5093-260-2764, U.S. Department of Energy Cooperative Agreement DE-FC21-86MC11076, and NSF-EPSCoR-ADP Fossil Energy Contract EHR-910-8774.

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