Abstract: Sequence Stratigraphy of the Muddy Formation at Hilight Field: Powder River Basin, Wyoming

Art D. Donovan

Detailed correlations of Lower Cretaceous strata in the vicinity of Hilight Field in the south-central Powder River Basin reveal that the Muddy reservoirs at Hilight are contained within a depositional sequence, bounded by an older unconformity and a younger unconformity.

Within this depositional sequence, three informal units: Upper Paralic (youngest), Lower Paralic, and Nonmarine (oldest), can be defined within the Muddy Formation, based on the correlation of two regional bentonite markers, herein termed M1 and M2. The basal Nonmarine Unit of the Muddy onlaps topography carved into the underlying Skull Creek Shale by the older unconformity, contains only nonmarine deposits in core, and is overlain by the M2 marker. This unit is interpreted as the lowstand systems tract of the depositional sequence. The overlying Lower Paralic and Upper Paralic units of the Muddy overlie the M2 and M1 markers respectively. These members contain shoreline and coastal plain deposits in core, and are interpreted as the highstand systems tract of the depositional sequence

Towards the margins of the Hilight Field, the Upper Paralic Unit, Lower Paralic Unit, and Nonmarine Unit are sequentially truncated by the younger sequence boundary. Thus Muddy thickness variations, spatial distributions, facies patterns, as well as the hydrocarbon trap at Hilight Field are related to both primary depositional trends, as well as secondary post-depositional (erosional and accommodation) patterns.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994