Gas Origin in Coals of the Blackhawk Formation, Castlegate Coalbed Methane Field, Utah

Martin Niemann¹, Paul R. Clarke², C. T. Cornelius², B. Ryan³, and M. J. Whiticar¹. (1) School of Earth and Ocean Sciences, Biogeochemistry Facility, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada, phone: 250 472 5006, [email protected], (2) Pioneer Natural Resources USA, Inc, 1410 17th Street, Suite 1200, Denver, CO 80202, (3) Ministry of Energy and Mines, 6th Floor, 1810 Blanshard St, P.O. Box 9326 Stn Prov Govt, Victoria, BC V8W 9N3, Canada

Coals of the Blackhawk Formation locally contain a world class in-situ methane resource base that is estimated to exceed 30 billion standard cubic feet of gas per sq mile. Following recent coring of the primary coal groups at the Castlegate CBM field, detailed carbon isotopic investigations of desorbed gases with mixed composition (C1, C2, C3, CO2) while strongly indicating a thermogenic origin, suggest that not all gas within the coal is internally sourced from humic kerogens. Rather, the isotopic signatures and gas composition indicate that the gas in place has a mixed origin from both internal and external sources, the later likely having being generated by type I & II kerogens from the underlying Mancos Shales, and adsorbed during uplift.

Detailed study of both subsurface and outcrop data, indicate that the migration pathway for secondary thermogenic enrichment is the likely combination of coal seam positioning relative to large progradational parasequences of the Spring Canyon, Aberdeen, Kenilworth and Sunnyside Members, and vertically connecting Laramide natural fracture networks. Collectively, the petroleum system of the Castlegate CBM field demonstrates the role that can be played by basin centered source rocks in enriching low to medium ranked coal groups.