The Origin of Natural Gas in the Tertiary Coal Seams on the Eastern Margin of the Powder River Basin

Abstract

Abstract The origin of methane being produced from Tertiary coals along the eastern outcrop margin of the Powder River Basin is bacterial. There is no evidence for a thermogenic origin of this gas originating either from Tertiary sediments, or from underlying Cretaceous gas reservoirs. The metabolic pathway used by bacteria to generate methane progressed via the reduction of CO2. The extremely dry composition of natural gas in these coals, and the stable isotopic composition of both carbon and deuterium isotopes in co-occurring methane, water, and CO2 support this conclusion. There are insufficient data to categorically determine the timing of bacterial methane generation in this basin. However, the available information suggests that methane was generated sometime following the onset of basin uplift, 10 to 35 mya., and possibly as recently as during the Pleistocene.