Coal as a source rock of petroleum and gas. A comparison between natural and artificial maturation by organic petrology and geochemistry: Example from the Almond Formation coals, Greater Green River Basin, Wyoming

Organic petrological and geochemical studies demonstrate that the Almond Formation coals in the Greater Green River Basin contain a great unrecognized volume of stored liquid petroleum as well as natural gas. Oil is generated during the maturation of hydrogen-rich vitrinite (desmocollinite) and liptinite macerals into exsudatinite (waxy oil) and inertinite solid residue. The waxy oil is initially stored in porus structures and subsequently in vesicles as the coal matures under increasing temperatures.;Primary migration of oil occurs as the generation of a sufficient volume of exsudatinite fractures the vitrinite semifusinite vesicles, interconnecting vesicles, and pores through microfracturing. Also, the thermal cracking of exsudatinite generates a sufficient volume of gas to fracture the vesiculated coal as pore pressure increases.;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 Basin at vitrinite reflectances between 0.9 and 1.7% indicate that these coals may have generated 24 billion barrels of oil and 66 trillion cubic feet of gas.