Stratigraphy and Hydrocarbon Potential of Muddy Sandstone, Wind River Basin, Wyoming: ABSTRACT

Abstract

The Lower Cretaceous (late Albian) Muddy Sandstone in the Wind River basin and adjacent areas rests unconformably upon the Thermopolis Shale and is disconformably overlain by and interfingers with the lower part of the Shell Creek Shale. The Muddy Sandstone is divided into five informal members: the Lazy B, Rozet, lower Recluse, upper Recluse, and Cyclone members. These members, defined as chronostratigraphic units bounded by bentonite beds, transgressive disconformities, and/or unconformities, consist of asymmetrical, coarsening and thickening-upward progradational deltaic/strand-plain and bay-fill deposits and/or asymmetrical, fining and thinning-upward aggradational valley-fill deposits. The Lazy B and Rozet members are marine progradational strand-plain deposits bounded by unconformities. They occur as discontinuous, erosional remnants in the western and northwestern parts of the study area. Following Rozet deposition, a major sea level fall resulted in the development of a headward-eroding valley system. During a subsequent sea level rise these valleys aggraded with fluvial/estuarine deposits of the Grieve member. Eventually the study area was blanketed by landward-stepping, progradational bay-fill, nearshore, and offshore marine deposits of the lower Recluse, upper Recluse, and Cyclone members. In addition to localized production from anticlinal or fault-bounded structural closure, the most significant oil and gas production is from updip pinch-outs ofmore » sand-rich valley fill of the Grieve member in the southeastern Wind River basin. Extensive outcrop and core analysis reveals these sand-rich paleovalleys, which range up to 10 mi wide and 100 ft thick, contain numerous scour-and-fill sequences composed of either fluvial bed load, fluvial suspended load, or abandoned channel deposits. Individual scour-and-fill sequences range up to 300 ft wide and up to 50 ft thick.« less