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Abstract: Fourbear Field: Immature Oil from a Permian Source Trapped in an Eocene Laccofold, Western Bighorn Basin


The Fourbear field is located on the Cody platform of the western Bighorn basin only a few miles from the eastern edge of the Absaroka volcanic plateau. This anticlinal oil accumulation is relatively shallow (reservoir depth; approx. 3000 ft; 914 m) and the common-pool Paleozoic reservoir is filled to the spill point (oil column = approx. 1300 ft, 396 m) with low gravity oil from a Permian Phosphoria source. Cumulative production of about 35 MMBO is estimated to be only 10% of the oil in place.

Core data from a crestal deep test, seismic, borehole, and potential fields evidence, indicate that the structural closure at Fourbear is created by a concordant, felsic, laccolithic body intruded into Upper Cambrian shales in the core of the anticline. The intrusion was probably injected through a basement fault zone into an immature, thrust-generated fold. To describe the elongate, doubly-plunging Fourbear structure, a new term, laccofold, is introduced.

Migration of immature, Phosphoria-sourced oil into the Paleozoic reservoir at Fourbear field could not have occurred until after intrusion of the laccolith in post-Laramide, Middle Eocene time (i.e., approx. 50 Ma). However, neither the identification of a discrete Phosphoria source rock, or a chronicle of oil generation and migration into the Fourbear trap is precisely constrained.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado