High Resolution Stratigraphy and Facies Architecture of the Upper Cretaceous (Turonian-Cenomanian) Eagle Ford Formation, Central Texas

The Upper Cretaceous Eagle Ford Formation has long been recognized as an important source rock for productive reservoirs throughout Texas, including sandstones of the Woodbine Fm. and limestones of the Austin and Buda Fms. Heightened industry focus on the Eagle Ford is a result of recent discoveries of producible unconventional petroleum resources in this emerging play. However, little has been published on the facies and facies heterogeneities within this mixed carbonate-clastic mudrock system. A rock-based study is fundamental to understanding the controls, types, and scales of inherent heterogeneities, which have implications for enhanced comprehension of the Eagle Ford Formation and other mixed carbonate-clastic mudrock systems worldwide.

In this study, we conducted a high-resolution analysis of facies distribution utilizing 10 subsurface cores along a one mile transect. These data were supplemented with selected outcrop and subsurface datasets. Facies definition and stratigraphic correlations were based on sedimentary structures, depositional textures, grain size, mineralogy, fossil assemblages and geochemical analysis. This study is further enhanced by synthesis of XRD, XRF, δ13C, and δ18O.

Eagle Ford deposition is characterized primarily by low energy hemipelagic sedimentation, bottom current reworking, and sediment gravity transport. Facies of the Eagle Ford include: 1) massive argillaceous clayshale, 2) massive argillaceous calcareous clayshale, 3) massive calcareous siliceous mudshale, 4) laminated calcareous siliceous mudshale, 5) laminated siliceous foraminiferal wackestone, 6) laminated foraminiferal packstone to grainstone, 7) thin bentonites, and 8) calcareous concretions. Globigerinid Foraminifera and nanoplankton constitute much of the variable carbonate content present within the facies.

Intra-formational variabilities within the Eagle Ford stem from the interplay of sediment input, deposition, bottom current reworking, burial, and diagenetic effects. This study provides high resolution insights into the small-scale variability of these heterogeneities, which, in turn, has direct implications on the large scale system. Understanding the controls, types, and scales of variability enhances the predictability and prospectivity of the Eagle Ford play, as well as other mixed carbonate-clastic mudrock systems.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California