Abstract

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 (depth of reservoir; ~3000 ft; 914 m) and the structural closure is filled to the spill point (vertical closure = oil column = ~1300 ft; 396 m) with immature oil (12-15° API gravity) from a Permian Phosphoria source. Cumulative production of about 35 MMBO is an estimated 10% of the oil in place.

The Fourbear structure appears to be a typical, northwest-trending Laramide anticline at the level of the Paleozoic common-pool reservoir, but in fact, is unique within the Wyoming foreland province. Core data from a crestal deep test, together with seismic support, show that the structural closure at Fourbear has been created by intrusion of a concordant laccolithic body into Upper Cambrian shale in the core of the anticline. It is proposed that this felsic, laccolithic intrusion was injected through a basement fault zone into an existing, immature, thrust-generated anticline. The intrusion probably began as a thin sill, expanding vertically and laterally to produce the elongate, doubly-plunging, Fourbear laccolith. The resulting structure is here described as a laccofold. The Fourbear lacolith and associated Absaroka volcanics are age-dated as Middle Eocene.

Migration of immature, Phosphoria-sourced oil into the Paleozoic reservoir at Fourbear field could only have occurred after intrusion of the laccolith in post-Laramide, Middle Eocene time (<50 Ma). But the identification of a discrete Phosphoria source rock (black shale or carbonate; distant or local?) and the time of primary oil generation and migration remain an enigma.