The Eagle Ford Outcrops of West Texas: A Laboratory for Understanding Heterogeneities Within Unconventional Mudstone Reservoirs

Arthur D. Donovan¹, T. Scott Staerker¹, Aris Pramudito¹, Weiguo Li¹, Matthew J. Corbett², Christopher M. Lowery³, Andrea Miceli Romero⁴, and Rand D. Gardner⁵
¹BP, 501 Westlake Park Blvd., Houston, Texas 77079, U.S.A.
²Department of Earth and Atmospheric Sciences, University of Nebraska–Lincoln, 214 Bessey Hall, P.O. Box 880340, Lincoln, Nebraska 68588, U.S.A.
³Department of Geosciences, University of Massachusetts, 611 North Pleasant St., 233 Morrill Science Center, Amherst, Massachusetts 01003, U.S.A.
⁴ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Sarkeys Energy Center, 100 E. Boyd St., Norman, Oklahoma 73019, U.S.A.
⁵Department of Geology and Geophysics, Texas A&M University, MS 3115, College Station, Texas 77843, U.S.A.

The Eagle Ford outcrops in West Texas provide a unique opportunity to examine strata equivalent to the Eagle Ford unconventional mudstone reservoirs of South Texas in excellent vertically and laterally continuous exposures. Whereas unconventional mudstone reservoirs are commonly portrayed as homogenous, our work to date reveals a vertically heterogeneous facies and total-organic-carbon (TOC) succession in the Eagle Ford Group, with variability at the bed-, parasequence-, sequence-, and sequence-set scale. This heterogeneity suggests the potential for distinct vertical variability in the unconventional reservoirs that would affect well performance. Understanding and predicting those variations is essential to enable effective horizontal-well placement in the subsurface.

Outcrops of the Eagle Ford Group in West Texas consist of a vertical succession of five distinct facies, each of which contains a vertical succession of sub-facies. These facies and sub-facies were used to divide the Eagle Ford Group into the Lower Eagle Ford Formation and Upper Eagle Ford Formation. Within the Lower Eagle Ford Formation a (lower) unnamed member and (upper) Middle Shale Member were defined, whereas in the Upper Eagle Ford Formation a (lower) unnamed member and the (upper) Langtry Member are proposed. Each of the four Eagle Ford members, which can be correlated from the outcrops of West Texas into the subsurface of South Texas, have distinct lithologic characteristics, geochemical signatures, geographic distributions, and chronostratigraphic significance. Because the four proposed lithostratigraphic members within the Eagle Ford Group are bounded by regionally mappable unconformities, they are also chronostratigraphic units that are not coeval to (1) each other, (2) the underlying Buda Limestone, or 3) the overlying Austin Chalk.

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012