Depositional systems analysis of the Lower Miocene Interval in Refugio County, Western Gulf of Mexico Basin

Definition of the environments within a depositional system provides useful information about the possible depositional processes; and in turn helps predict the amount and caliber of sediments transported to the basin. This research analyzes variance attribute maps to identify the different environments of deposition within Refugio County, Texas; this analysis also addresses the possible influence by the San Marcos Arch on Lower Miocene deposition. The study area is subsurface, Lower Miocene strata of Refugio County situated in the western GOM basin. Numerous variance attribute maps were generated from a three dimensional (3D) seismic volume. These maps reveal that the stratigraphic section is predominantly an expanded regressive phase. The basal Miocene strata that immediately overlie the Anahuac Shale preserve the record of significant shoreline progradation as shown by a thick and laterally extensive complex of amalgamated beach-ridge deposits associated with longshore transport of sand. These beach deposits are overlain by a thick section dominated by incised valleys fills, and channel and channel-belt deposits. Subtle change in incised valley shape is interpreted to record change in distance or relative proximity to the shoreline. The logs from 17 wells are integrated with the 3D seismic data to quantify sandstone/shale variability and develop sand maps. The San Marcos Arch is a significant structural feature located towards the northeastern part of the study area. Contoured sand thickness maps of four intervals within the dataset indicate increase in sand thicknesses towards the northeast, indicating that the influence of the San Marcos Arch on sediment deposition had waned by the Lower Miocene.