The Oligocene Frio Formation, totaling approximately 15,000 ft, is an important oil and gas reservoir on the South Texas Gulf Coast. More than 70 trillion cubic ft of gas and 8 billion barrels of oil have been produced from the Frio—mostly from upper Frio sandstones and more recently from deeper, lower Frio sandstones. If systems tract concepts are used, additional reserves are likely to be found in new downdip plays; therefore, an understanding of the regional sequence stratigraphy is necessary and critical.

Six third-order sequences have been recognized within the Frio depositional sequence. Each sequence consists of basin-floor and slope-fan deposits and a prograding wedge deposited off the shelf during a third-order sea-level lowstand. Growth faults were active during lowstand sedimentation and ceased their influence with rising sea levels, when transgressive (TST) and highstand (HST) systems tracts shifted shelfward. The shifting of the depocenter in time, in conjunction with fault activity, created a strong volumetric partitioning between the slope and basin and the shelf. Because genetically similar but noncontemporaneous lowstand (LST) depositional systems successively filled each subbasin, this sequence-based approach proved to be useful in unraveling the complex stratigraphic architecture of the Frio subbasins.

This study focuses on three subbasins: Encinal (Corpus Christi), Red Fish, and Mustang Island, over a four-county area in the South Texas coastal area, integrating wireline-log, micropaleontologic, seismic, core, and reservoir data. We reconstructed individual systems tracts for each of the six sequences within the three subbasins that are displayed in cross sections and isopach maps.