Evaluation of the depositional environment of the Eagle Ford Formation using well log, seismic, and core data in the Hawkville Trough, LaSalle and McMullen counties, south Texas

Abstract

The Upper Cretaceous Eagle Ford Formation of South Texas records a mixed siliciclastic/carbonate depositional environment across the Late Cretaceous Platform of the Gulf of Mexico. During the Late Cretaceous, LaSalle and McMullen Counties was part of the Hawkville Trough, a wedged shaped region between the Edwards and Sligo carbonate reefs. Well logs from 21 wells and seismic data were used to construct structure and isopach maps of the Eagle Ford Formation throughout the Hawkville Trough. Only the unconformable bottom (Buda-Eagle Ford) and top (Eagle Ford-Austin Chalk) boundaries plus the conformable boundary between the upper and lower Eagle Ford can be consistently correlated in the area. The Eagle Ford-Austin Chalk boundary is variable/gradational due to variable erosion of the Eagle Ford prior to deposition of the Austin Chalk. This variability is also observed in core data. The Lower and Upper Eagle Ford are trough shaped deposits that strike northeast roughly parallel with the Edwards reef. Maximum thickness of the Lower Eagle Ford is more than 180 ft in LaSalle County and 140 ft in McMullen County. The Upper Eagle Ford has a maximum thickness of 160 ft along the LaSalle-McMullen County Border. Both the Lower and Upper Eagle Ford thin to half their maximum thickness within 5-6 miles of the axis. Depth to the top of the Eagle Ford varies from 9600 ft to 15000 ft and strikes parallel to the Edwards and Sligo reefs. Numerous faults are visible. Most faults are post-depositional with modest offsets. Few faults are syn-depositional growth faults and the Eagle Ford is thicker on the down thrown side. Well and seismic data document dramatic decreases in thickness of the Upper Eagle Ford over a few miles. In the most extreme case in southwest LaSalle County, the Upper Eagle Ford is entirely missing in 2 wells and has been replaced by a sand unit not previously reported. A seismic cross section, or time slice, just above the top of the Eagle Ford shows a channel structure running west to east along southern LaSalle County. This channel is likely the cause of observed erosion and sand deposition.