Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
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
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
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.