Reservoir geology, structural architecture, and sequence stratigraphy of a growth-faulted subbasin : Oligocene Lower Frio formation, Red Fish Bay area, South Texas Gulf Coast

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Summary

An integrated study using 3D seismic, wireline logs, and core analyses was conducted to establish new strategies for exploring compartmentalized, lowstand, prograding deltaic systems. The Frio sili...

An integrated study using 3D seismic, wireline logs, and core analyses was conducted to establish new strategies for exploring compartmentalized, lowstand, prograding deltaic systems. The Frio siliciclastics, totaling approximately 11,000 ft., are commercial gas reservoirs in many of the growth-faulted, intraslope subbasins in South Texas. Red Fish Bay field is located in such a growth-faulted subbasin. Some Frio reservoir sandstones in Red Fish Bay field are part of a Frio third-order, lowstand prograding-wedge tract. This third-order systems tract is composed of 10 higher-order, lowstand deltaic, and superposed transgressive systems tracts. Many of the reservoirs in the higher-order lowstand deltas and transgressive sandstones are composed of fine-grained, lithic arkoses that have mean porosities of 20 to 25 percent and mean permeability in the tens of millidarcys. 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 incipient growth faults, which were present at weaknesses of the slope, displaced highly mobilized muds basinward of the growing lowstand sedimentary wedge. The growth fault, active during lowstand sedimentation, ceased its influence with rising sea level and the shifting of sedimentation shelfward. Regionally, growth faults trend generally northeast-southwest, setting up small subbasins. Associated with the growth faults are numerous subparallel, postdepositional synthetic faults. Some normal faults trend perpendicular to the growth faults, establishing a complex pattern of fault compartmentalization that dissects the prograding-wedge depositional patterns. Pressure-decline analysis demonstrates compartmentalization that is due to (1) lateral discontinuous sandstone bodies and (2) sandstone bodies that have several pressure compartments defined by fault segregation.

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