Coal as a source rock of petroleum and gas - a comparison between natural and artificial maturation of the Almond Formation coals, Greater Green River Basin in Wyoming

Organic petrological and geochemical studies demonstrate that the Almond Formation coals contain great unrecognized volumes of stored gas and oil. Oil is generated during maturation of hydrogen-rich vitrinite (desmocollinite) and liptinite macerals into exsudatinite (waxy oil) and inertinite solid residue. The waxy oil is initially stored in pores and vesicles. As the coal thermally matures, stored hydrocarbons are expelled from the pores and vesicles. This phase change causes a significant volume increase, which may overcome the storage capacity of these coals, fracturing them and allowing primary migration of hydrocarbons. Kinetic modeling, based on hydrous pyrolysis experiments, indicates that at the basin center, most oil generated and expelled from Almond coals has been thermally cracked to gas, whereas at the basin flank the oil-to-gas reaction is unimportant. During hydrous pyrolysis these coals expel up to 0.17 barrels of oil and 404 cubic feet of gas per ton of coal, indicating excellent generative capacity. Calculations of the volume of Upper Cretaceous coals in the Greater Green River at vitrinite reflectances between 0.9 and 1.7 percent indicate that these coals may have generated 24 billion barrels of oil and 66 trillion cubic feet of gas. 39 refs., 51 figs., 11 tabs.