Datapages, Inc.Print this page

Integrating Outcrop and Subsurface Data to Define Regional and Reservoir-Scale Patterns in Prograding Systems, Lewis Shale and Fox Hills Sandstone, Wyoming

Pyles, David 1; Slatt, Roger M.2
1 Chevron Center of Research Excellence, Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO.
2 School of Geology and Geophysics, University of Oklahoma, Norman, OK.

Prograding shelf-slope-basin clinoforms are oil and gas reservoirs in many parts of the world such as West Siberia Basin, northwest slope of Australia, North Slope of Alaska. The Cretaceous Lewis Shale and Fox Hills Sandstone of the Greater Green River Basin, Wyoming is an excellent outcrop analog for these systems. These formations contain clinoforms greater than 400 meters in relief that record the progradation of a linked shelf-slope-basin system. Outcrops and well logs are used to quantitatively describe reservoir architecture in seven physiographically distinct areas: (1) fluvial, (2) shelf, (3) shelf edge,
4 slope, (5) proximal base-of-slope, (6) medial base-of-slope, and (7) basin.

Fluvial strata are ~25% sandstone and are composed of channels, coals, splays, and overbank strata. Shelf strata are ~30 % sandstone and are composed of marine shale, distributary mouth bars, distributary channels. Shelf-edge strata are >60% sandstone and are composed of distributary channels, river-mouth bars, marine shale, 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 ~30% sandstone and are composed of mudstone and thin-bedded sandstone that is locally truncated by submarine channels. 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. Basin floor strata are 20% sandstone and are composed of distal turbidite lobes.

A quantitative comparison of the data reveals that ~75% of the sandstone in the entire depositional system are partitioned between two physiographically distinct areas: shelf edge and medial base of slope. These data further reveal that the diversity of reservoir elements and facies decreases from the fluvial system to the basin floor. Furthermore, each physiographic position is distinguishable in terms of net:gross, architectural associations, facies associations, and gamma-ray patterns.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009