Stra­tigraphy and oil and gas resources in uppermost Cretaceous and Paleocene rocks, Wind River Reservation, Wyoming

Abstract

The Cody Shale and the Mesaverde, Meeteetse, and Lance Formations of Late Cretaceous age and the Fort Union Formation of Paleocene age within the Wind River Reservation contain strata that were deposited during the final major regression of the Cretaceous epicontinental sea eastward across central Wyoming and the ensuing initial stages of mountain-building and basin subsidence of the Laramide orogeny. The Reservation spans several major structural elements in the western part of the Wind River Basin, but the feature of primary importance to evaluations of future petroleum resource potential is the western end of the deep basin syncline, which occupies the east-central and southeastern parts of the Reservation where many thousands of feet of synorogenic deposits accumulated. The Cody Shale is characterized by 3,300-4,000 ft of marine shale and sandstone, the latter rock type predominating in the upper part of the formation and grading upward into the basal, regressive sandstone units of the Mesaverde Formation. The Mesaverde, Meeteetse, Lance, and Fort Union Formations are primarily of fluvial origin, and consist mostly of interbedded sandstone and shale with various amounts of carbonaceous shale and thin coal beds. Maximum thicknesses of these formations are 2,150 ft, 1,370 ft, 2,900 ft, and 6,200 ft, respectively. Some parts of the full sequence may be partially or totally cut out beneath erosional unconformities at the base of either the Lance or Fort Union Formations, or at the base of the lower Eocene rocks, near the basin margins. All of the uppermost Cretaceous and Paleocene rocks have yielded commercial quantities of hydrocarbons (chiefly natural gas), primarily from closed anticlines such as the Pavillion and Muddy Ridge fields, but in part from apparent stratigraphic traps formed by the updip pinchouts of lenticular sandstones along the west and southwest margins of the basin syncline as well as within the basin proper. Drilling is sparse in these areas, and the potential for stratigraphic entrapment has yet to be explored in extensive parts of the Reservation.