Sequence Architecture and Structural Setting of a Growth-Faulted Subbasin, Frio Formation, South Texas

Ursula Hammes, Robert G. Loucks, L. Frank Brown, Ramon H. Trevino, Randy L. Remington, and Patricia Montoya
Bureau of Economic Geology, The University of Texas, Austin, Texas

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, Texas. The wedge resulted from a drop in relative sea level that caused a basinward shift of facies. The fluvial systems were locked into incised valleys and loaded the underlying distal slope and basinal fine-grained sediments with coarser grained sediments. Slope failure generated a growth faulted subbasin and later, a basinward shale ridge. Subsequent sea- level rise terminated the faulting. Coarsening upward log facies suggest lowstand deltaic sedimentation within a prograding wedge. Individual sands within similar adjacent subbasins do not correlate. Diachronous deltaic sandstones pinch out basinward and display potential updip rollover or fault closure. Transgressive sandstones exhibit landward onlap terminations.

Deposition occurred adjacent to the hanging wall of a major growth fault system. Two southeast-dipping growth faults compose the western and northern basin boundaries. A post-depositional fault orthogonal to the western boundary did not influence prograding wedge deposition. Syndepositional fault movement, rollover, and associated interval expansion occurred in the prograding wedge section of the Frio Formation. Hanging wall, crestal rollover faults are a major control on trapping and compartmentalizing hydrocarbons. Production and petrophysical analyses of prograding wedge reservoirs indicate gas-expansion drive north of the post-depositional, orthogonal fault and water drive south of it. 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.

AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004