Abstract

A bentonite-rich mudstone facies occurs within the lower half of the Eagle Ford Group in the outcrop belt, as well as in shallow boreholes, along the western flank of the East Texas Basin. The continuity of this facies along the outcrop belt suggested to some that this unit might have chrono-stratigraphic significance, and utility for correlations in the deeper subsurface across the East Texas Basin. An early attempt to utilize this concept, however, appeared to fail when this bentonite-rich zone was absent in a core that spanned the entire Eagle Ford Group in Quitman Field (Wood County, Texas) about 100 miles to the east of Dallas.

Recently, this bentonite-rich zone was identified as an unconformity-bounded sequence, and defined as the Upper Member of the Lower Eagle Ford Formation (UM:LEF). As part of a regional study of the Woodbine and Eagle Ford groups in the northwestern East Texas Basin, observations from shallow boreholes (Fig., 1) and X–ray fluorescence (XRF) analysis (Fig. 2) were used to construct and correlate a grid of well log cross sections. These correlations revealed that the organic-rich Lower Eagle Ford Formation, as well as the bentonite-rich, upper member within it, could be defined and mapped within the study area. These correlations revealed that the Lower Eagle Ford Formation is sequentially truncated from the top down, both to the north and east, by a regional unconformity located at the base of the overlying Upper Eagle Ford Formation (Fig. 3). Thus, in the vicinity of Quit-man Field in Wood County, the Upper Eagle Ford Formation is unconformably juxtaposed on top of the Woodbine Formation, and the Lower Eagle Ford, as well as its bentonite-rich upper member, is absent due to truncation (Fig. 4). Besides solving the “mystery of the missing bentonites,” this work defined the limits of the play fairway for the Lower Eagle Ford unconventional reservoirs in this portion of the basin.