Sequences, Depositional Systems, and Synsedimentary Tectonics, Post-Eocene Rocks, Corpus Christi Region, South Texas: Emphasis on Oligocene Frio and Anahuac Geologic History

L. Frank Brown, Jr., Robert G. Loucks, and Ramón H. Treviño: Bureau of Economic Geology

Abstract

Integration of regional 3-D seismic data sets and wireline logs allowed us to define second to fifth order (~10 my-10 ky) Frio and Anahuac (Oligocene) sequences, systems, and syntectonics. All third and some fourth order sequences were correlated within a regional-scale network. Vicksburg and Miocene sequences were of secondary interest. Composite sequence logs characterized principal fields. Microfossil data were limited.

Lithostratigraphic Frio and Anahuac strata comprise six chronostratigraphic, third order depositional sequences (~32.0-23.6 Ma) and myriad fourth and fifth order sequences or parasequence sets. Except for incised valley fills, lowstand tracts comprise off-shelf systems deposited within active growth-faulted, intraslope subbasins. Maximum Anahuac flooding (~24.6 Ma) provides a regional marker. Condensed sections and type 1 unconformities are isochronous. Sand-rich facies are mostly diachronous. Off-shelf and on-shelf deposition are temporally unique. Most previously inferred Frio "stacked barriers" are incised valley-fill facies.

Seaward, lowstand sedimentary wedges and superposed shelves became younger. Entrenched rivers and ephemeral deltas supplied sediments for gravity transport to basin floors and slope fans. Overloaded depocenters initiated gravity faulting, mud ridges, and, hence, younger subbasins. Diminished faulting resulted in lowstand deltas completely filling the subbasins. Highstand shorelines then prograded basinward over stabilized, lowstand ramps until they stalled in deepening water near continental shelf edges.

Gas in lowstand deltaic and distal valley-fill reservoirs is trapped updip against hanging walls of extensional faults overlying shale-cored anticlines. Combination trapping and basin-floor and slope-fan reservoirs are viable targets. Sequence stratigraphic ideas offer new prospecting options in mature basins.