Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
invited luncheon presentation for San Antonio Chapter of the Society of Independent Professional Earth Scientists (SIPES), September 16, 2004
Understanding Growth-Faulted, Intraslope Subbasins and Associated Reservoir Targets by Applying Sequence Stratigraphic Principles: Examples from the South Texas Oligocene Frio Formation
Robert G. Loucks, L. Frank Brown, Jr., Ramón H. Treviño, and Ursula Hammes
Detailed analysis of Oligocene Frio Formation intraslope growth-faulted subbasins in the Corpus Christi, Texas, area indicates that deposition during relative lowstands of sea level was the main initiator, or "trigger," of growth faulting. Lowstand depocenters on the low-gradient upper continental slope comprising basin-floor-fan facies, slope-fan systems, and prograding lowstand delta systems exerted sufficient gravity stress to trigger major sections of outer shelf and upper slope strata to fail and move basinward. The faults sole out deep in the basin, and rotation of hanging-wall blocks mobilized deep-water muds and forced the mud basinward and upward to form mud (shale) ridges that constitute the basinward flank of intraslope subbasins overlying footwall fault blocks.
Lowstand sedimentation associated with third-order falls of relative sea level produced load stress that triggered major regional syndepositional growth-fault systems. Subbasins on the downthrown side of each arcuate fault segment composing a regional fault system were filled during a single lowstand of sea level. Consequently, genetically similar but diachronous lowstand depositional systems filled each successive growth-faulted subbasin trend. The subbasin development and fill extended the Frio shelf edge stepwise into the Oligocene Gulf of Mexico Basin. Thus each successive, basinward subbasin was younger than the previous landward subbasin.
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. Off-shelf and on-shelf deposition are temporally unique. 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.
Sequence-stratigraphic analyses of Oligocene (Frio Formation) growth-faulted subbasins in Corpus Christi Bay and offshore Mustang Island demonstrate that current exploration targets compose sand-rich, proximal, deltaic, prograding wedge, and incised-valley-fill sandstones, respectively. Postdepositional crestal faults on rollover anticlines provide reservoir trapping mechanisms. Wireline-log facies of productive reservoirs in the subbasins are genetically similar, but more than 10 mi (> 15 km) apart, and several major faults separate successive subbasins. A methodology is presented that incorporates the sequence-stratigraphic interpretation of each subbasin, which improves correlations of systems tracts between the widely separated subbasins. This methodology consists of composite wireline logs created by splicing unfaulted and relatively conformable log segments from the deepest wells in an area. The composite log provides a stratigraphic record that captures a complete succession of depositional and cyclic history. Site-specific sequence-stratigraphic-section (S5) benchmark charts contain composite logs and additional data that summarize available geologic information for a specific subbasin.
Growth-faulted subbasins all along the Texas coast have been prolific petroleum targets for decades and are now the focus of prospecting for deep, on-shelf gas. Lowstand basin-floor and slope-fan sandstones are the principal gas targets. Understanding the origin of the growth-faulted subbasins and their chronostratigraphic relationships and depositional processes provides a perspective that can improve deep on-shelf exploration.