Abstract

Approximately 48 percent (more than 659 million barrels) of Wyoming's oil production has come from fields in the Powder River Basin. Much of the oil has been produced from Cretaceous strata where updip loss of relative permeability is the primary factor controlling accumulation.

Significant differences in the composition of the crude oils are found to be associated with the original depositional facies of the source rocks but other factors such as chemical "weathering," catalytic activity, effect of meteoric waters, association with unconformities, and depth of burial were effective in altering their chemical composition. Average compositions for oils thought to be indigenous to Cretaceous, Jurassic, possibly Triassic, Permian and Pennsylvanian strata are established.

In the past the term "Tensleep" has been applied inappropriately to Converse or Upper Minnelusa strata of Early Permian age in the Powder River Basin. It is demonstrated that the oil contained in these reservoirs is different not only from that thought to be indigenous to Pennsylvanian strata, but also to that of the true Tensleep and Phosphoria oils of the Big Horn Basin.

Oil accumulation in the Powder River Basin may have occurred episodically throughout geologic history as a result of local differential deformation, but the present principal entrapment conditions resulted from relatively recent gravitational adjustments to a final "Laramide" structural configuration except as modified by hydronamic effects.