New Insights Into Regional Sequence Stratigraphic Correlations of the Oligocene Frio Formation in South Texas

Ursula Hammes, David L. Carr, Robert G. Loucks, and Frank L. Brown
Bureau of Economic Geology, University of Texas, Austin, TX

The Frio section, totaling approximately 1,500 ft, contains commercial gas reservoirs in several growth-faulted, intraslope subbasins in South Texas. More than 70 Tcf of gas and 8 Bbbl of oil have been produced from the Frio—mostly from Upper Frio sandstones and more recently from deeper, Lower Frio sandstones. Using system tract concepts, additional reserves are likely to be found in new downdip plays; therefore, an understanding of the regional sequence stratigraphy is necessary and critical.

Many papers have described individual fields in the Frio Formation; however, a detailed regional, integrated, sequence stratigraphic framework study is lacking. A refined and reinterpreted regional sequence stratigraphic framework has therefore been constructed. We have focused our study on four counties in the South Texas coastal area by integrating wireline-log, micropaleontologic, seismic, core, and reservoir data. In the project area, the Frio was deposited in six consecutive growth-faulted subbasins related to third-order, sea-level cycles. We mapped individual system tracts within the sequence fairway to account for depositional patterns, sand fairways, contemporaneous faulting, and reservoir hydraulics. Each sequence consists of off-shelf, lowstand systems tracts, which comprise deep density-flow deposits (basin-floor and slope fans) upward through prograding lowstand deltaic wedge complexes. Growth faults were active during lowstand sedimentation and ceased their influence with rising sea levels, when transgressive and highstand systems tracts shifted shelfward. An understanding of the subbasin structure and paleontologic framework in concert with the sequence stratigraphic architecture will improve prediction of stratigraphic and areal distribution of deeply buried lowstand reservoir systems and traps.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas