My focus of study is the nature of sediments within late Eocene to middle Oligocene, coast-perpendicular subbasins located on the southwest side of the San Marcos Arch in Refugio County, near the South Texas Gulf Coast. Methods of investigation consist of 3D seismic interpretation, seismic attribute, and isochron mapping. Depositional geometry, inferred sediment composition, and bed-thickness data are examined by extraction of amplitude-envelope, sweetness, and thin-bed-indicator (TBI) attributes from the 3D seismic volume. Four subbasins, numbered SB1 through SB4 from northeast to southwest, are recognized. Although the coast-perpendicular subbasins formed at the same time and by the same processes, the sediment distribution, basin-fill history, and tectonics differ from subbasin to subbasin within the Jackson, Vicksburg, and Frio Formations. Amplitude envelopes extracted at base Jackson and at near-base Vicksburg Formation show that, although the distribution pattern of interpreted sandstone-prone sediments within each subbasin is mainly of sheet geometry, SB2 displays a pattern of short, narrow, linear features. Based on available well control, the sweetness and TBI attributes correlate well with Vclay log. Crossplots between Vclay and sweetness exhibit a strong, linear, negative relationship — the sweetness value increases as the Vclay value decreases, suggesting an increasing sandstone content. In contrast, crossplots between Vclay and TBI exhibit a positive linear relationship — the TBI value increases as the Vclay value increases, suggesting an increasing shale content. Because the sweetness attribute may in some cases not detect some thin shale bed within a stratigraphic interval, it may overestimate the net:gross ratio. Therefore, a combined TBI and sweetness procedure is required to determine the net:gross ratio. The results show that the net:gross ratio decreases from SB1 closest to the San Marcos Arch, to SB4 farthest into the Rio Grande Embayment. The approach adopted in this study can be used to estimate net:gross ratios in frontier areas.

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