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The Eagle Ford Outcrops of West Texas: A Laboratory for Understanding Heterogeneities, As Well As Sequence Stratigraphic Controls, on Unconventional Mudstone Reservoirs

Donovan, Arthur D.; Staerker, T.Scott; Pramudito, Aris; Corbett, Mathew J.; Lowery, Christopher M.; Romero, Andrea M.; Gardner, Rand

The Eagle Ford outcrops in West Texas provide a unique opportunity to examine unconventional mudstone reservoirs in excellently exposed vertically and laterally continuous outcrops. Whereas unconventional mudstone reservoirs like the Eagle Ford are commonly portrayed as being homogenous reservoirs, our work to date reveals a vertically heterogeneous facies and TOC succession with variability at the bed-, parasequence-, sequence-, and sequence-set scale. This heterogeneity suggests the potential for distinct vertical variability in unconventional reservoir type affecting well performance. Understanding and predicting those variations is essential to enable effective horizontal well placement in the subsurface.

The outcrops of the Eagle Ford Group in West Texas consist of 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 a vertical succession of two distinct formations and four unique members. 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, reservoir properties, geographic distributions, and chronostratigraphic significance. Since the four proposed lithostratigraphic members of the Eagle Ford Group are bounded by regionally mappable unconformities, these stratigraphic units are not coeval to: 1) each other, 2) the underlying Buda, or 3) the overlying Austin. Understanding the thickness and distribution the various Eagle Ford Members is a powerful tool for explaining and predicting the distribution of the best reservoir facies in the subsurface of South Texas.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013