From Bureau of Economic Geology, The University of Texas at Austin (
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2003 STGS/GCSSEPM One-day Research Conference, San Antonio, Texas, April 11, 2003

Revisiting Mature Fields with Modern Technology and Geologic Concepts: Examples from the Frio of South Texas

L. Frank Brown, Jr., Robert G. Loucks, Ramón H. Treviño


Subregional 3-D seismic volumes and wireline logs permitted definition of second- to fifth-order (~10 my-10 ky) Frio and Anahuac (Oligocene) sequences, systems tracts, and associated syntectonics. Third- and most fourth-order sequences were correlated within several subregional wireline-log and seismic networks. Vicksburg and Miocene sequences were of secondary interest. Composite sequence logs (figure 1) characterize principal fields. Sequence analysis identified and correlated all key surfaces: type 1 unconformities, maximum flooding surfaces, and transgressive surfaces bounding systems tracts. Although microfossil occurrences are not necessarily required for sequence analysis, limited data were integrated with the final sequence frameworks, providing secondary verification of assigned ages.

Lithostratigraphic Frio and Anahuac strata comprise six chronostratigraphic, third-order depositional sequences (~32.0-23.38 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 (~24.57 Ma) provided a regional, dated marker to which latest published ages of sequence surfaces were calibrated. Maximum flooding surfaces and type 1 unconformities are essentially isochronous, but sand-rich lithofacies are mostly diachronous. Off-shelf and on-shelf deposition were temporally unique. Many previously inferred Frio "stacked barriers" are dip-oriented incised valley-fill facies.

Seaward, lowstand sedimentary wedges and superposed shelves become younger. Entrenched rivers supplied sediments via ephemeral deltas for gravity transport to basin floors and slope fans. Eventually, overloaded lowstand depocenters initiated gravity faulting, mobilized mud, and, hence, produced younger faulted, shale-withdrawal subbasins. Diminished faulting permitted lowstand deltas to extend shelf edges basinward until the deltaic ramps were anchored at the basinward margin of buried subjacent shale ridges. These shale buttresses stabilized the upper continental slope and shelf edge. During a later cycle, highstand shorelines prograded basinward over the shallow, lowstand ramps. On-shelf regression eventually stalled by increasing accommodation space near the continental shelf edge, establishing another depocenter and intraslope subbasin.

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 ideas offer new, but deeper prospecting targets.