Stratigraphy of the Lower Cretaceous Muddy Formation and the Drainage Pattern of Its Paleovalleys, Wind River Basin, Wyoming

VAN DEN BOLD, M. C., BP Exploration, Houston, TX, and S. R. JOHNSON and E. R. GUSTASON, BP Exploration, Anchorage, AK

Valley-fill deposits of the Lower Cretaceous Muddy Sandstone contain significant stratigraphically trapped hydrocarbon accumulations and are an important exploration target in Wyoming's Bighorn, Powder River, and eastern Wind River basins. An integrated subsurface and outcrop study of the Muddy Sandstone in the western Wind River basin was conducted in order to determine (1) if Muddy paleovalleys are present there, (2) if potential for stratigraphic traps exists, and (3) if source rocks of sufficient quality and maturity are able to charge the Muddy reservoirs.

The results of the study reveal that the Muddy Sandstone (up to 100 ft thick) contains a series of unconformity bounded progradational shoreface deposits, separated by a valley-fill sequence (up to 60 ft thick). In general, the Muddy Formation is divided into three informal zones: (1) a lower marine zone, containing at least two progradational shoreface highstand parasequences; (2) a middle zone, which consists of a lowstand fluvial/estuarine valley-fill sequence (8-10 mi wide); and (3) an upper marine zone, containing at least three landward-stepping highstand shoreface parasequences.

In the southwestern Wind River Basin, the Muddy paleovalley trend is north-northwest, parallel to the present-day structural trend of the Wind River thrust and its parasitic anticlines. However, instead of continuing into the potentially prospective northwestern Wind River basin, the paleovalley makes an abrupt 90 degrees turn westward, a few miles north of Ft. Washakie. This trellis drainage pattern is characteristic of Muddy paleovalleys throughout Wyoming and is believed to reflect recurrent syndepositional movement along southwest-northeast-trending basement faults. A detailed understanding of how Muddy paleovalley drainage reacts to basement faults could become a key predictive tool for future Muddy exploration in the northern Rocky Mountains.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)