Potential for commercial hydrocarbon accumulations in Eocene and older strata of the Duval County Ranch in South Texas was assessed using seismic data. Three seismic facies are recognized and tied to well control. Seismic facies 1 has low- amplitude, continuous seismic reflections; seismic facies 2 has short, discontinuous, high-amplitude reflections; and seismic facies 3 has long, continuous, intermediate to high-amplitude reflections. In the upper Wilcox Group (lower Eocene) and Queen City Formation (middle Eocene), seismic facies 1 and 2 are thought to be sandstone-rich coastal/delta-plain and delta-front/distributary-mouth-bard deposits, respectively, and they both rank high in exploration potential. Seismic facies 3 is thought to be a mudstone-rich, deeper water shelfal deposit having lower exploration potential. The Duval County Ranch area was a locus of clastic deposition during the early Eocene because of subsidence and growth faulting along the underlying Wilcox Fault Zone. Sandstone-rich seismic facies 1 and 2 of the upper Wilcox Group are thus widespread in the Duval County Ranch area. Shelf-margin progradation followed upper Wilcox deposition, and by Queen City time, coastal/delta-plain deposits of seismic facies 1 dominated the Duval County Ranch area. Seismic facies 2 and 3 of the Queen City Formation are restricted to the southeasternmost part of the study area.
Seismic character of the Yegua (upper middle Eocene) and Jackson (upper Eocene) stratigraphic intervals resembles that of the underlying upper Wilcox Group and Queen City Formation. Seismic facies 1, 2, and 3 are recognized within the Yegua and Jackson over the Duval County Ranch area, but only seismic facies 1 is widespread within the boundaries of the Duval County Ranch. Although seismic facies 1 is associated with sandstone-rich barrier-island and strandplain deposits in the Jackson-Yegua interval, prospectivity of this seismic facies is more difficult to assess than that of the underlying units because, within this interval, hydrocarbon accumulation depends heavily on trapping mechanisms provided by stratigraphic pinch-out and facies variability. Unlike the upper Wilcox, effects of the Wilcox Fault Zone on Jackson-Yegua strata were minor, now represented only as small-scale faults and gentle flexures.
In all stratigraphic units, seismic facies 1 and 2 are more prospective when they contain specific horizons having very high amplitude "bright spot" reflections. A bright spot anomaly in Queen City sandstones produces oil and gas in Lundell field, and untested bright spot anomalies detected in this study appear elsewhere in the Queen City Formation and Jackson Group. High-amplitude seismic reflections of Jackson sandstones to the southeast of Seventy-Six West field suggest potential for a field extension or new field discovery. Upper Wilcox seismic facies 2 exhibits numerous untested zones in the Duval County Ranch area that are expressed as discontinuous, very high amplitude reflections. These random bright spot anomalies are associated with a zone of expanded upper Wilcox section in which sand deposition was focused on the downthrown side of the growth faults where rollover structures provide closure. A trend of greatly expanded upper Wilcox section, known as the Deep Wilcox Trend, occurs principally downdip of the Duval County Ranch, and potential for discovery of fields such as Seven Sisters East or Rosita Northwest is restricted to the southeasternmost corner. We also discovered that the lower Wilcox appears to be sandstone poor in the Duval County Ranch area, and we therefore assigned it a low exploration potential. Below the lower Wilcox, a high-risk deep gas exploration play, comprising large structures associated with salt withdrawal, lies in the Lower Cretaceous.
Fiduk, J. C., and Hamilton, D. S., 1995, Seismic Analysis of the Duval County Ranch Area, South Texas: Assessment of Exploration Potential of the Wilcox, Queen City, and Jackson-Yegua Plays: The University of Texas at Austin, Bureau of Economic Geology, Geological Circular 95-4, 42 p.