Cave Gulch field, Natrona County, Wyoming: Large gas discovery in the Rocky Mountain foreland, Wind River basin

Cave Gulch field in the Wind River basin of Wyoming is an important new sub thrust gas discovery area with reserves of 500 bcf to 1 tcf. Production com es from multiple reservoir intervals ranging in age from Paleocene to Early Cretaceous, with additional potential in underlying Mesozoic and Paleozoic units. Subsurface well data and recent three-dimensional seismic interpret ation indicate that the productive structure consists of a complexly faulte d anticlinal high beneath the leading detachment of the Owl Creek thrust, a major zone of basement reverse faulting that bounds the Wind River basin t o the north and east. Wells in the field produce gas at rates of 1-3 bcf ga s/yr and have reserves commonly in the range of 10-20 bcf. Since its discov ery in 1994, Cave Gulch has been most intensively developed in fluvial sand stones of the Paleocene Fort Union and uppermost Cretaceous Lance formation s. Both intervals exhibit a combination of primary and secondary porosity, with the Lance Formation also showing a significant degree of fracturing. S tructure controls productivity within the field; pay distribution within th e Fort Union and Lance formations is closely dependent on position relative to the anticlinal. crest and to related faulting. Accurate seismic interpr etation in this area has proved challenging because of strong lateral veloc ity variations associated with the thrusted basement at relatively shallow levels. Solutions of related problems, as well as the impressive size of th e accumulation, have likely stimulated new interest in exploration associat ed with the Laramide basement uplifts of the Rocky Mountain region.