Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Baylor University Seminar, February 18, 2005
Gas Reservoir Compartmentalization in Lowstand Prograding-Wedge Deltaic Systems: Oligocene Upper Lower Frio Formation, South Texas
Ursula Hammes, L. Frank Brown, Jr., Ramón Treviño, Randy Remington, Bob Loucks, and Patricia Montoya
An integrated study with 3-D seismic, logs, and core analyses was conducted to establish new strategies for exploring in compartmentalized, lowstand, prograding deltaic systems. The Frio sandstones, totaling approximately 1,500 feet, are commercial gas reservoirs in the growth-faulted, intraslope subbasins along the South Texas Gulf Coast. The upper lower Frio third-order lowstand deltaic-wedge sequence is composed of ten forth-order lowstand deltaic and superposed transgressive depositional systems tracts. Main reservoirs in lowstand and transgressive deltaic, fine-grained, lithic arkoses have a mean porosity of 20 percent and mean permeability of 40 mD. Incised rivers contributed large amounts of sediment to the continental slope via shelf-edge, ephemeral, high frequency, lowstand deltas located at notched river mouths. Basin-floor fan deposition shifted to slope-fan processes as the rate of relative falling sea level decreased. These subbasinal slope fans aggraded and onlapped the upper slope. Lowstand depocenters eventually overloaded unconsolidated mud-rich, water-saturated sediments. Gravity failure along the upper slope generated syn-depositional faults that displaced highly mobilized muds basinward of the growing lowstand sedimentary wedge. These growth faults trend generally northeast-southwest, parallel to the coast line, setting up small subbasins. Associated with the growth faults are numerous subparallel, post-depositional synthetic faults. In addition, there are numerous normal faults that trend perpendicular and orthogonal to the growth faults setting up a complex pattern of fault compartments, which dissect the prograding-wedge depositional patterns. Pressure-decline analysis demonstrates compartmentalization that is due to (1) laterally discontinuous sandstone bodies and (2) a common sandstone body that has several pressure compartments defined by fault segregation.