Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Bureau of Economic Geology Technical Seminar, September 19, 2003
Sequences, Depositional Systems, and Synsedimentary Tectonics, Oligocene Rocks, Corpus Christi Region, South Texas: Revisiting Mature Fields with New Prospecting Tactics
L. Frank Brown
Subregional 3-D seismic volumes and wireline logs permitted recognition of 2nd- to 5th-order (~10 m y-10 k y) 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. Composite site specific sequence stratigraphic sections (S5 benchmark logs) 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 (~32.0-23.6 Ma) and myriad fourth- and fifth-order sequences or parasequence sets. Except for incised valley fills, lowstand tracts comprise offshelf systems deposited within active, growth-faulted, intraslope footwall subbasins. Maximum Anahuac 2nd-order flooding (Heterostegegina sp.condensed section) provided a regional, age-dated (~24.57 Ma 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. A major error in previous studies has been the assumption that all sands deposited between successive maximum flooding surfaces must be correlative. In fact off-shelf and on-shelf deposition were temporally unique. Many previously inferred Frio strike aligned "stacked barriers" (i.e., blocky on wireline logs) are dip-oriented incised valley-fill facies, but wave-dominated highstand and transgressive tracts are characterized by strike-aligned blocky barrier/strandplain systems that were eroded by incised valleys.
Basinward of shelf edges, successive lowstand sedimentary subbasin wedges and superposed shelves become younger. Entrenched rivers supplied sediments via ephemeral deltas until high gradients induced gravity failure and transport to basin floor and slope fans. By mid-slope-fan deposition load-stressed lowstand depocenters initiated gravity faulting, mobilized deep muddy facies, and, hence, produced younger faulted, shale-withdrawal subbasins that experienced abnormally high accommodation rates. Later diminished rates of faulting and accommodation permitted lowstand deltas to prograde over slope fans and shelf edges farther basinward. Lowstand deltaic ramps were anchored at the basinward margin of buried subjacent shale-ridge that established the subsequent upper continental slope and shelf edge. During a later cycle, highstand shorelines regressed basinward over the shallow, lowstand ramps. On-shelf regression was eventually stalled by increasing accommodation space and deeper water near the continental shelf edge, followed by creation of another lowstand depocenter and intraslope subbasin. .
Gas is trapped in lowstand deltaic and superposed thin transgressive parasequences and distal valley-fill reservoirs that pinchout updip against hanging-wall extensional faults overlying shale-cored anticlines. Combination trapping and basin-floor and slope-fan reservoirs are viable targets. Sequence results support improved traditional and improved speculative prospecting methods.