From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, April 29, 2005
Sequence Architecture and Structural Setting of a Growth-Faulted Subbasin, Frio Formation, South Texas
An integrated study, which included 3-D seismic, logs, and core analyses, was conducted to establish new strategies for exploring in compartmentalized, lowstand, prograding deltaic systems. The Frio sandstones are commercial gas reservoirs in growth-faulted, intraslope subbasins along the South Texas Gulf Coast. Much of the Frio Formation was deposited as fourth- and fifth-order components of a third-order, lowstand, prograding deltaic wedge within and across growth-faulted subbasins in the Red Fish Bay area. The wedge resulted from a drop in relative sea level that caused a basinward shift of facies. The lower part of the off-shelf lowstand comprises basin-floor fan deposition, which subsequently shifted to slope-fan processes as the rate of relative sea-level fall decreased. These subbasinal slope fans aggraded and onlapped the upper slope. Upward-coarsening log facies suggest lowstand deltaic sedimentation within a prograding wedge overlying basin and slope sediments. Lowstand depocenters eventually overloaded unconsolidated, mud-rich, water-saturated sediments. Gravity failure along the upper slope generated syndepositional faults that displaced highly mobile muds basinward of the growing lowstand sedimentary wedge. These growth faults trend generally northeast-southwest, parallel to the coastline, setting up subbasins. Higher frequency, fourth-order, sea-level fluctuations initiated even smaller minisubbasins with sedimentary and fault patterns similar to those of the larger-scale, third-order subbasins. Associated with the growth faults are numerous subparallel, postdepositional synthetic faults. In addition, normal faults 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. The best producing intervals are three- to four-way closures in low-resistivity, fourth-order lowstand and transgressive sands within an overall third-order, lowstand prograding wedge.