Abstract

Production trends and reservoir decline curves indicate that current Wyoming oil reserves have a "half-life" of approximately 5 years. Therefore, the search for unconventional sources of petroleum, as well as unconventional reservoir types, becomes increasingly more important in a semimature producing province such as the Greater Green River Basin of Wyoming. Two unconventional exploration strategies, both well suited to application in the Greater Green River Basin, focus on the concepts of coal as a source rock of liquid (as well as gaseous) petroleum and abnormally pressured, hydrologically compartmentalized reservoirs. These concepts are investigated in geochemical studies and experiments that evaluate the upper Cretaceous Lance Formation coals in terms of their capacity to generate liquid and gaseous hydrocarbons, the timing of gas generation that may have caused coal fracturing and the release of generated oil, and the establishment of a multiphase fluid flow system and concomitant establishment of sealed compartmentalized reservoirs.

As of 1992, the Mesaverde Group of the Greater Green River Basin (GRB) had produced 9 trillion cubic feet of gas and 400 million barrels of oil, accounting for 11% of the cumulative production in Wyoming from only 5% of its wells. Previous studies of Almond Formation coals associated with the Cretaceous Mesaverde Group have shown them to be prolific sources of natural gas as well as liquid petroleum. Inasmuch as the Lance Formation coals are geochemically and petrographically similar to the Almond Formation coals, information on the timing of generation, storage, and expulsion of liquid and gaseous hydrocarbons from Lance coal is of importance to the understanding of hydrocarbon accumulations in the GRB. Data from the current study, derived from hydrous pyrolysis experiments on coals from the lance Formation, have established kinetic parameters (activation energies and frequency factors) for Ahrrenius-type calculations of first-order kinetic reactions. These kinetic parameters are coupled with geochemical, kinetic, and geohistory parameters to produce information on the timing of the generation and expulsion of oil from the coals, the oil-to-gas reaction, the coal-to-gas reaction, and water-rock, organic-inorganic interactions that may affect reservoir quality.

It is likely, however, that the products of the thermal maturation of the coals do not easily migrate until the coals are fractured. One mechanism for fracturing the coals in release of pressure built up from the evolution of gas during thermal maturation, provided this evolution is rapid enough that the gases haven't time to dissipate. Plots of the first derivative (rate of change) of generation curves for the two main gases produced by the coals during thermal evolution (CO₂ and CH4) show that there are four distinct pulses of gas from the coals.

A significant portion of the gas produced from the Cretaceous section is from overpressured reservoirs in tight gas sands. The pressure regime of many of these reservoirs includes abnormally pressured, capillary sealed compartments that appear to have formed during the oil-to-gas reaction. This reaction establishes a multiphase fluid flow system and activates capillary seals in low-permeability rocks. Plots of the first derivative of the generation and expulsion curves for oil and gas maturation reactions against time and temperature yield estimates of the minimum timing of sealing of these compartments with hydrocarbons, and thus of the establishment of this type of hydrocarbon reservoir.