From Bureau of Economic Geology, The University of Texas at Austin (
For more information, please contact the author.

SIPES Seminar, Houston, September 30, 2004

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, and Ursula Hammes


[Schematic diagram in pdf format]

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. One third-order Frio sequence comprises on-shelf and basinal fourth-order sequences. Composite sequence logs (S5) characterize principal fields. Sequence analysis identified and correlated all key surfaces: e.g., type 1 sequence-bounding unconformities, maximum flooding surfaces within marine-condensed sections, and transgressive surfaces bounding systems tracts. Although microfossil data 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 On-shelf facies are components of strike-oriented highstand and transgressive Frio “stacked barrier”systems and lowstand dip-oriented incised valley-fill systems.

Seaward, lowstand slope and deltaic wedges and superposed on-shelf systems become younger. Entrenched lowstand rivers supplied sediments via ephemeral deltas for gravity transport to basin floors and slope fans. Eventually, overloaded lowstand depocenters triggered 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, hence, establishing another depocenter and intraslope subbasin.

Gas in lowstand deltaic and distal valley-fill reservoirs is trapped updip against extensional faults overlying shale-cored anticlines within hanging-wall blocks . Combination trapping and basin-floor and slope-fan reservoirs are viable Lowstand targets. Understanding sequence architecture and inferred lowstand depositional processes improves the odds in prospecting for by-passed and deeper targets in maturely drilled trends that were explored exclusively by searching for conventional structural closures.