--> Abstract: Integrating Outcrop and Subsurface Data to Define Regional and Reservoir-Scale Patterns in the Lewis Shale and Fox Hills Sandstone of the Great Divide and Washakie Basins, Wyoming, by D. R. Pyles and R. M. Slatt; #90092 (2009)

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Integrating Outcrop and Subsurface Data to Define Regional and Reservoir-Scale Patterns in the Lewis Shale and Fox Hills Sandstone of the Great Divide and Washakie Basins, Wyoming

David R. Pyles1 and Roger M. Slatt2
1Department of Geology, Chevron Center of Research Excellence, Golden, CO
2School of Geology and Geophysics, University of Oklahoma, Norman, OK

The Cretaceous Lewis Shale and Fox Hills Sandstone of the Great Divide and Washakie Basins, Wyoming are a significant gas resource in the Rocky Mountains. Regional studies in the Lewis Shale have revealed southward facing clinoforms that resulted from the progradation of a linked shelf-slope-basin system. Outcrops along the eastern margins of the basins were integrated into a regional cross section in order to resolve how facies and stratigraphic architecture changes in four physiographically distinct areas: (1) shelf edge, (2) slope, (3) proximal base-of-slope, and (4) medial base-of-slope. These outcrops provide a unique opportunity to resolve regional and reservoir-scale patterns in this complex system.

Shelf-edge strata are >50% sandstone and are composed of channels, river-mouth bars, mudstone sheets, and large sandstone slumps. Slumps appear to be related to seafloor instability at the shelf edge. These deposits are interpreted to record a mechanism for generating sediment gravity flows that transmitted sandstone to the slope and base of slope positions. Slope strata are only ~15-20% sandstone and are composed of mudstone that is locally truncated by submarine channels that display architectural and facies asymmetry. This asymmetry is interpreted to reflect channel sinuosity. A large proportion of the mudstone in slope strata is interpreted to be levee strata. Proximal base-of-slope strata are ~50% sandstone and are composed of sandy submarine-fan strata consisting of slumps, amalgamated submarine channels, and turbidite lobes. Medial base-of-slope strata are >80% sandstone and are composed entirely of turbidite lobes. This area is the sandiest part of the depositional system.

The patterns described can be used to help interpret borehole image and core data for Lewis Shale and Fox Hills Sandstone reservoirs.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado