Abstract Subregional 3-D seismic volumes and wireline logs permitted definition of second- to fifth-order (~10 my–10 ky) Frio and Anahuac (Oligocene) sequences, systems tracts, and associated syntectonics. Third- and most fourth-order sequences were correlated within several subregional wireline-log and seismic networks. Vicksburg and Miocene sequences were of secondary interest. Composite sequence logs ( Figs. 1 and 2 ) characterized principal fields. Sequence analysis identified and correlated all key surfaces: type 1 unconformities, maximum-flooding surfaces, and transgressive surfaces bounding systems tracts. Although microfossil occurrences are not necessarily required for sequence analysis, limited data were integrated with the final sequence frameworks, providing secondary verification of assigned ages.

Abstract An integrated study using 3D seismic, wireline logs, and core analyses was conducted to establish strategies for exploring in compartmentalized, lowstand, prograding, deltaic systems. Frio sediments, averaging ~11,000 ft, are commercial gas reservoirs in several of the growth-faulted, intraslope subbasins in South Texas. These exploitation targets are typically located in fault-bounded compartments that form three- and four-way structural closure. However, understanding the stratigraphic component is crucial to successful exploitation of these sandstones. The Frio third-order lowstand prograding wedge in Red Fish Bay Field is composed of 10 higher order, lowstand deltaic and superposed transgressive, depositional systems tracts. The sandstones are fine-grained, lithic arkoses having a mean porosity of 20% and tens of millidarcies of permeability. The main constituents of these sandstones are quartz, feldspar, and volcanic-rock fragments. Feldspars typically exhibit a substantial amount of secondary dissolution and micropores. Quartz cements, as well as interstitial clays, are minor. Porosity reduction occurs primarily by compaction. Gravity failure along the upper slope generated syndepositional faults that displaced mobilized mud basinward of a 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, normal faults trend perpendicular to the growth faults, establishing a complex pattern of fault compartmentalization, which dissects the prograding-wedge depositional patterns. Pressure-decline analysis demonstrates compartmentalization that is due to (1) laterally discontinuous sandstone bodies and (2) fault-segregated sandstone bodies.