The Oregon Basin oil and gas field is in north-central Wyoming on the western margin of the Big Horn basin, a closed Tertiary-filled structural basin. Adjacent to the synclinal axis of the asymmetrical Big Horn basin, Oregon Basin is a large anticline having 1,600 feet of closure. Transverse tensional and shear faulting is present along the axis, both in the surface and subsurface strata. The oil and gas accumulation is present primarily because of this structural trap. The anticline is divided into two domes separated by a low saddle which is considered unproductive.

The folding responsible for the present structural configuration began in late Upper Cretaceous time. Subsequent movement has occurred during the Tertiary.

The most highly developed and important production is black, 20°-22° gravity oil from the Permian Embar limestone and the Pennsylvanian Tensleep sandstone. This productive area occupies only 40 per cent of the total area within the closing contour. Black, 18° gravity oil is also produced from the Mississippian Madison limestone. Commercial amounts of gas occur in the Cretaceous Frontier and Cloverly sandstones and the Triassic Chugwater sandstones. Gas is present in commercial quantities both separately and in association with the oil in the Embar limestone. Oregon Basin is one of the more important reserves of black oil in Wyoming, having an estimated ultimate production in excess of 150,000,000 barrels.

A well on the south dome has penetrated a normal sequence of strata from the surface to the pre-Cambrian granite. No production was found below the presently known Madison oil zones. A non-commercial showing of light paraffine-base oil was found in the basal Cambrian Flathead sandstone. The limits of the present production have been mostly defined and little exploratory drilling is foreseen for the future.