Architecture and variability of Lower Wilcox sand prone slope deposits, South Central Texas

Abstract

This study uses a large3D seismic survey (1,200 km2) and sparse well data to reveal the large-scale slope geometries of the Lower Wilcox in south-central Texas. A new workflow applies a tree-based machine learning algorithm to predict relative impedance, which is in turn used to map sand volume (thickness) and net-gross ratio. The process proves a significant improvement over standard amplitude mapping and seismic facies analysis, leading to a better analysis of architecture and variability of Lower Wilcox sand prone slope deposits in South Central Texas. After Midway transgression deltas of the Lower Wilcox prograded under high stand conditions of sea level over a shallow wide shelf. River dominated wave and tide-influenced deltas on the inner shelf provided sandstone via hyperpycnal flows to the upper slopes. Muddy sediments with normal and inverse grading and low borrowing intensities and diversities observed in a 40 m core reflect deposition on a mud dominated shelf controlled by hyperpycnal processes. There are substantial sand accumulations in intraslope minibasins formed by growth fault development. Accommodation of the growth strata is created by deformation of the underlying slope muds, which are inferred to have had high initial porosities, and thus were easily mobilized. Evidence of syn-sedimentary faulting comes from channel diversion and intraslope ponding of flows with laterally amalgamated channels and lobate sand sheets. Alternation between slope core channel and lobe deposition is caused by variability in the magnitude of hyperpycnal activity. At seismic resolution, a high-frequency cycle accumulates about 200 m sediments in average. Sand filled upper slope channel complexes range in size from 200 m to more than 1 km wide. Stacked channel sand thickness ranges from 10 to 50 m. Lobes spread 10-20 km laterally and 2-4 km downdip, with a maximum total sand thickness of 100 m and maximum net-gross ratio of 0.5. At depocenter, accumulated sand thickness in multiple high- frequency units is as high as 500 m.