Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
American Association of Petroleum Geologists Annual Meeting, Dallas, April 18-21, 2004
Gas Reservoir Compartmentalization in Lowstand Prograding-Wedge Deltaic Systems: Oligocene Upper Lower Frio Formation, South Texas
Ursula Hammes, L. Frank Brown, Ramón Treviño, Randy Remington, Robert G. Loucks, and Patricia Montoya
An integrated study using 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 several of the growth-faulted, intraslope subbasins in South Texas. The upper lower Frio third-order lowstand deltaic-wedge sequence is composed of ten fourth-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. Lowstand depocenters eventually overloaded unconsolidated mud-rich, water-saturated sediments. Gravity failure along the upper slope generated syndepositional faults that displaced highly mobilized muds basinward of the growing lowstand sedimentary wedge. These growth faults trend generally northeast-southwest, setting up small subbasins. Associated with the growth faults are numerous subparallel, postdepositional synthetic faults. In addition, there are 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.