Depositional History of the Upper Wilcox Group and Lower Reklaw Formation, Northern Bee County, Texas

William A. Ambrose

Bureau of Economic Geology
Jackson School of Geosciences
The University of Texas at Austin

Abstract

A detailed study of the upper Wilcox and lower Reklaw stratigraphic succession in a ~190-mi2 (490-km2) area along the upper Wilcox shelf margin in northern Bee County, Texas, resolves these units into a series of 19 fourth-order sequences and demonstrates that the upper Wilcox to lower Reklaw succession contains significant variability in depositional systems, facies, and reservoir sandstone-body geometry than previously documented. Although wave-modified, fluvial-dominated deltaic systems are inferred in the upper Wilcox succession from previous studies in which facies tracts and depositional systems are inferred from thick (commonly >400-ft [>122-m]) stratigraphic intervals that encompass several fourth-order sequences, this study documents a wide variety of other depositional systems such as wave-dominated shoreface, inner-shelf, lower-coastal-plain streamplain, and lowstand fluvial systems. This study further demonstrates the need to divide the upper Wilcox succession into high-resolution fourth-order sequences in order to better infer controls on facies and sandstone-body geometry on reservoir distribution and productivity at a fine scale. 

A complex shoreline trajectory in the upper Wilcox to lower Reklaw succession records net coastal offlap, punctuated by numerous transgressive-regressive cycles representing shoreline retreat and advance. The lower one-half of the upper Wilcox succession represents a major, 700-ft (213-m) retrogradational cycle composed mostly of deltaic and shoreface deposits. This retrogradational cycle, capped by shelf deposits that represent maximum coastal onlap, is overlain by a 300-ft (91.5-m) regressive cycle that culminated in a period of forced regression associated with lowstand incised-valley fill deposits. The shoreline trajectory in the uppermost upper Wilcox section records another cycle, 150 to 200 ft (45 to 60 m thick) of transgression and shoreline retreat, followed by a final, 100- to 150-ft (30- to 45-m) phase of coastal offlap. In contrast, the overlying lower Reklaw stratigraphic succession represents a period of shoreline stabilization with a series of aggradational shoreface depositional intervals along the downthrown side of a major growth fault in northern Bee County along the upper Wilcox/lower Reklaw shelf margin.

The upper Wilcox succession in northern Bee County, Texas includes a prominent valley-fill system within the Luling Sand, a composite of a lower, shallow-marine, shoreface system truncated by an upper incised-valley-fill fluvial system (Figs. 1 to 3). The lower Luling Sand consists of barrier-core and barrier-margin (backbarrier) deposits in a wave-dominated shoreline setting (Figs. 2 and 4). Modern depositional analogs for wave-dominated coastal deposits in lower Luling Sand core include the wave-dominated, microtidal, and transgressive shoreline near the Santee Delta and Cape Romain in South Carolina, where the sandy coastline pinches out landward into muddy backbarrier and transgressive washover-fan and destructional-beach facies.

In contrast, the upper Luling Sand represents lowstand, incised-valley deposits that truncate shallow-marine deposits in the lower Luling Sand (Figs. 1 and 3). Diagnostic features for this incised-valley system include (1) inferred truncation of underlying strata (Fig. 1), aggradational wireline-log responses commonly characterized by blocky gamma-ray (GR) and spontaneous potential (SP) curves, and straight-to-braided gross-sandstone patterns in map view (Fig. 3). Differences between sandstone-body geometry and architecture between the upper and lower Luling Sand are a function of unique depositional origin and have implications for future reservoir development in northern Bee County and adjacent areas in south-central Texas. Detailed knowledge of sandstone-body geometry and reservoir continuity in each system (fluvial versus shallow-marine, upper versus lower Luling Sands, respectively) can be used for strategically targeting additional areas for infill and stepout wells where combination structural-stratigraphic traps may exist.

Publication approved by the Director, Bureau of Economic Geology.
 

Biography
William A. Ambrose

William A. Ambrose is a Research Scientist at the Bureau of Economic Geology. He received a Master of Arts degree in geological sciences in 1983 from the University of Texas at Austin. Since joining the Bureau of Economic Geology in 1987, he has worked on a variety of projects at the Bureau, including characterization of the Woodbine Group in the East Texas Basin, Frio fluvial and deltaic reservoirs in South Texas, tight-gas reservoirs in the Cleveland Formation in the Texas Panhandle, co-production of gas and hot brine from Oligocene reservoirs in the Texas Gulf Coast, evaluation of coalbed methane reservoirs in Rocky Mountain basins, and reservoir characterization and basin analysis studies in Venezuela and Mexico. He is currently the principal investigator of the Bureau’s STARR (State of Texas Advanced Oil and Gas Resource Recovery) program, past president of the Energy Minerals Division (EMD) of AAPG, chair of the EMD Coal Committee, and past co-chair of the AAPG Astrogeology Committee. His contact information is--email: william.ambrose@beg.utexas.edu , telephone: 512-471-0258, address: Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, TX, 78713-8924.