Submarine Mass Transport Complex Evolution and Control on Ooverlying Reservoir Geometry, Permian Cutoff Formation, West Texas

Robert Amerman¹, Eric P. Nelson¹, Michael H. Gardner², and Bruce Trudgill^1
1 Colorado School of Mines, Golden, CO
² Montana State University, Bozeman, MT

The Williams Ranch Member of the Cutoff Formation consists of six offlapping, basinward-stepping lithologic units of highstand carbonate turbidites deposited across a drowned Early Permian carbonate platform, then partially redistributed in slumps on the slope and basin floor. Slumps are intercalated with undeformed carbonate turbidites; the ratio of slumps to undeformed sediment increases basinward. Upslope evacuation scars correlate to downslope slump bodies. Gravity flow deposition and subsequent mass movement caused basinward thickening of the Williams Ranch Member and caused the toe of slope to shift 5 km basinward relative to the underlying Bone Spring Limestone. This shift controlled landward pinchouts of the overlying Permian Brushy Canyon Formation channel and sheet sandstone bodies.

Williams Ranch Member deposition both responded to and modified inherited bathymetric relief. Williams Ranch isopach thicks correspond to larger underlying lows and smaller overlying highs and exhibit a higher ratio of undeformed to slumped sediment and a higher percentage of soft-sediment folds relative to soft-sediment truncation surfaces. These slump “pile-ups” appear to be concentrated in inherited lows. Slump bodies show a general southward transport vector, with significant local variation possibly reflecting underlying bathymetric influence. With repeated slump events, “pile-up” zones resulted in local positive bathymetry. Brushy Canyon sand fairways and ponded sheets are focused in bathymetric lows, and sands are sidelapped against highs atop the Williams Ranch Member. Increased understanding of mass transport complex evolution may lead to better prediction of overlying reservoir geometry, both within the Brushy Canyon Formation and in analogous reservoirs in other deepwater settings.