Abstract

Cave Gulch field in the Wind River basin of Wyoming is an important new subthrust gas discovery area with reserves of 500 bcf to 1 tcf. Production comes 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 interpretation indicate that the productive structure consists of a complexly faulted anticlinal high beneath the leading detachment of the Owl Creek thrust, a major zone of basement reverse faulting that bounds the Wind River basin to the north and east. Wells in the field produce gas at rates of 1-3 bcf gas/yr and have reserves commonly in the range of 10-20 bcf. Since its discovery in 1994, Cave Gulch has been most intensively developed in fluvial sandstones of the Paleocene Fort Union and uppermost Cretaceous Lance formations. Both intervals exhibit a combination of primary and secondary porosity, with the Lance Formation also showing a significant degree of fracturing. Structure controls productivity within the field; pay distribution within the Fort Union and Lance formations is closely dependent on position relative to the anticlinal crest and to related faulting. Accurate seismic interpretation in this area has proved challenging because of strong lateral velocity variations associated with the thrusted basement at relatively shallow levels. Solutions of related problems, as well as the impressive size of the accumulation, have likely stimulated new interest in exploration associated with the Laramide basement uplifts of the Rocky Mountain region.

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