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Detailed stratal correlations of the Skull Creek, Muddy, Shell Cheek, and Mowry formations in the vicinity ofHilight field in the south-central Powder River basin, Wyoming, reveal that the Muddy Sandstone in this region is an unconformity-bounded stratigraphic unit (depositional sequence). This depositional sequence represents an erosional remnant of what were originally more widespread incised-valley and younger (high- stand) shoreline deposits.

The unconformity at the base of the Muddy Sandstone (green sequence boundary) is characterized by incision ofunderlying Skull Creek strata and onlap of overlying Muddy strata. In core, this boundary also corresponds to anabrupt basinward shift in fades (coastal plain on distal offshore deposits) and, locally, rooted horizons.

The unconformity at the top of the Muddy Sandstone (red sequence boundary) is characterized by truncation of and incision into underlying Muddy strata, onlap of overlying Shell Creek strata, and angular discordance. In core, this sequence boundary coincides with a marine flooding surface that is locally underlain by root structures.

Within the “Muddy” depositional sequence (green to red sequence boundaries) three informal units, nonmarine (oldest), lower paralic, and upper par- alic, can be defined. These units are separated by two regional well-log markers (Ml and M2) that, in core, correspond to bentonites that overlie interpreted marine flooding surfaces. The basal nonmarine unit of the Muddy: (1) onlaps topography carved into the underlying Skull Creek Shale by the green sequence boundary, (2) contains only nonmarine deposits in core, and (3) is overlain by a flooding surface and the M2 marker. This stratigraphic interval is interpreted as the lowstand systems tract (incised-valley fill) of the green sequence. The overlying lower paralic and upper paralic units of the Muddy contain shoreline and coastal-plain deposits in core and are interpreted as the highstand systems tract of the green sequence. Shell Creek strata which onlap and overlie the red sequence boundary are interpreted as the lowstand and transgressive systems tracts of the red depositional sequence. Bentonites and organic-rich shales at the base of the overlying Mowry Shale represent the condensed section of the red depositional sequence.

Toward the margins of the Hilight field, the upper paralic, lower paralic, and nonmarine units of the Muddy are progressively truncated by the over- lying red sequence boundary. Thus, both primary depositional and secondary erosional patterns control Muddy thickness variations, spatial distributions, facies patterns, and the hydrocarbon trap at Hilight field.

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