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

Abstract

A detailed study of the upper Wilcox Group and lower Reklaw Formation, based on core, wireline-log, and 3-D seismic data in northern Bee County, Texas resolves this succession into 19 fourth-order sequences and demonstrates greater variability in depositional systems, facies, shoreline evolution, and reservoir sandstone-body geometry than previously documented. Earlier studies of the upper Wilcox Group in south Texas interpreted fluvial-dominated, wave-modified deltaic systems from thick (commonly >400-ft [>122-m]), undivided stratigraphic intervals that encompass multiple depositional episodes. In contrast, mapping thinner, fourth-order sequences reveals a mosaic of wave-dominated shoreface, inner-shelf, lower-coastal-plain streamplain, and fluvial systems. A complex shoreline trajectory records numerous transgressive-regressive cycles representing multiple episodes of shoreline retreat and advance in south Texas. The lower one-half of the upper Wilcox succession represents a major, 700-ft (213-m) retrogradational cycle capped by shelf deposits. It is overlain by a 300-ft (91.5-m) regressive cycle including a bedload fluvial system that truncates wave-dominated shoreline deposits. In turn, the overlying lower Reklaw stratigraphic succession represents a period of shoreline stabilization along the upper Wilcox/lower Reklaw shelf margin. Although upper Wilcox sedimentary delivery systems were continental in scale, most upper Wilcox sequences in northern Bee County are composed of small-scale depositional elements inferred to occur between regional, large-scale depocenters. Brazos Delta and other small-scale depositional features such as tidal inlets and lower-coastal-plain streamplain systems are appropriate analogs for upper Wilcox and Reklaw sequences at local scales in south Texas and should be considered in additional reservoir development.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018