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Abstract: Depositional Environment of Lower Cretaceous Muddy Formation of Grieve Field, Wyoming

Gary C. Mitchell

Grieve field, in the southeastern Wind River basin of Wyoming, has produced over 29 million bbl of oil since being discovered in 1954. The trap is formed by an abrupt undip pinchout of a thick Muddy sandstone along the east flank of the anticlinal Rattlesnake Mountains. Traps in nearby Muddy fields are similar to the Grieve trap.

The Muddy Formation in Grieve field is interpreted as an estuarine deposit through core studies, subsurface mapping, palynology, and micropaleontology. Cores from areas of thicker sand accumulations contain a diagnostic vertical sequence of grain size, sedimentary and biogenic structures, and lithologies. Floral and faunal data differentiate marine and nonmarine sediments. Subsurface mapping established the geometry and stratigraphic relations of the Muddy Formation. Excellent modern analogs to the Grieve estuarine model are the estuaries of the Rhine delta in the Netherlands.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado