W. A. Ambrose, Hongliu Zeng, Jinyu Zhang, M. I. Olariu, David Smith, and Sigrid Clift
This detailed study of the upper Wilcox and Reklaw stratigraphic succession in a 190-mi2 (~490-km2) area along the upper Wilcox shelf margin in northern Bee County, Texas, interprets and maps 19 high-frequency, fourth-order, regressive–transgressive sequences. It demonstrates that the upper Wilcox to Reklaw succession contains greater variability in depositional systems, facies, and reservoir sandstone-body geometry than previously documented. Earlier studies of the upper Wilcox succession interpreted primarily fluvial-dominated, wave-modified deltaic systems from thick (commonly > 400-ft [> 122-m]), undivided stratigraphic intervals that encompass multiple depositional episodes. In contrast, this study documents a greater variety of depositional systems that include wave-dominated shoreface, transgressive shoreline, inner-shelf, lower-coastal-plain streamplain, and fluvial. It also depicts significant variability in the shoreline geometry and position in the upper Wilcox to Reklaw succession.
Although upper Wilcox sedimentary delivery systems were continental in scale, this study demonstrates that many upper Wilcox sequences in northern Bee County are composed of small-scale depositional elements that occur between regional depocenters extending beyond the 190-mi2 (~490-km2) study area. It also demonstrates that facies analogs such as the Brazos Delta and other small-scale depositional features such as tidal inlets and lower-coastal-plain streamplain systems along the Texas Gulf Coast are appropriate analogs for upper Wilcox and Reklaw sequences in northern Bee County and, by extension, for other areas in the south-central Texas Gulf Coast.
Structural controls on deposition for upper Wilcox and Reklaw sequences in northern Bee County are inferred from abrupt increases in both interval thickness and net-sandstone values across growth faults. Abrupt increases in net-sandstone values coincide commonly with facies changes, including the transition from lower-coastal-plain to shore-zone and from distributary-channel to channel-mouth-bar/proximaldelta-front facies. Strike-parallel shore-zone systems in most Reklaw sequences are preferentially developed on the downthrown side of upper Wilcox shelf-edge fault systems, suggesting shoreline stillstands controlled by increased accommodation related to differential subsidence.
Trap styles for hydrocarbons in the upper Wilcox and Reklaw stratigraphic succession include structural, stratigraphic, and combination structural/ stratigraphic types. The most common productive facies in wave-dominated shoreline and deltaic systems in both the upper Wilcox Group and Reklaw Formation in northern Bee County occur in shoreface/ beach facies, whereas distributary-channel facies are predominant in fluvial-dominated deltaic systems. Sandy channel-axis facies in fluvial systems in the upper Luling Sand in the upper Wilcox Group are the most common in terms of production. Although a total of 43 of the 195 wells in the study (~22%) have a history of oil and gas production, the current distribution of producing wells in many upper Wilcox and Reklaw sequences is limited, suggesting that other areas in northern Bee County may have potential for additional oil and gas production.
About This Publication
Our work is a detailed study of upper Wilcox and Reklaw stratigraphy in a 190-mi2 (~490-km2) area along the upper Wilcox shelf margin in northern Bee County, Texas. By interpreting and mapping 19 sequences,we show that this area contains greater variability in depositional systems, facies, and reservoir sandstone-body geometry than was previously documented.
Bee County, depositional systems, oil and gas production, Reklaw Formation, sequence South Texas, stratigraphy, Texas, Wilcox Group
Ambrose, W. A., Zeng, H., Zhang, J., Olariu, M. I., Smith, D., and Clift, S., 2018, Depositional History and Stratigraphic Evolution of the Upper Wilcox Group and Reklaw Formation, Northern Bee County, Texas: The University of Texas at Austin, Bureau of Economic Geology Report of Investigations No. 284, 87 p.