Stratigraphy and Facies Characterization of the Cenomanian to Turonian Eagle Ford Group in Southwest Texas: Implications for Identifying Potentially Productive Hydrocarbon Pay Zone

Abstract

The upper Cretaceous (Cenomanian to Turonian) Eagle Ford Group is one of the most productive unconventional plays in the world. In the United States, the play extends from the Maverick Basin in south Texas (at the U.S. – Mexico border) northeastward into the east Texas Basin. Along the depositional trend, the Eagle Ford varies both in thickness and stratigraphy due to changes in structural and depositional settings, making this prolific unconventional play highly heterogeneous and consequently difficult to predict productive hydrocarbon pay zones prior to extensive drilling. The study is based on core and well log data from wells in Karnes, McMullen, Live Oak and Atascosa counties. The Eagle Ford Group in these wells is divided into facies based on a combination of depositional and diagenetic characteristics such as lithological variations identified in cores, skeletal grain types and abundance observed in thin-sections, bulk rock carbon and oxygen stable isotope analysis for depositional conditions and diagenetic alterations, and XRF data for subtle chemostratigraphic variations. The core description study indicates that the main lithological facies are skeletal wackestone-packstone, foraminiferal packstone, mudstone and bioturbated mudstone, whereas the XRF data indicate that the main chemofacies are siliceous marl, pyritic mudstone, calcareous marlstone, argillaceous marlstone, calcareous mudstone and siliceous mudstone. These lithofacies boundaries correlate well with the chemofacies boundaries. These facies are correlated with petrophysical well log data, Rock-Eval analysis and measured total organic carbon (TOC) for identification and characterization of potential productive hydrocarbon pay zones. Accordingly, the well log data, Rock-Eval and TOC data may provide detailed understanding of the thickness and stratigraphic variations of the organic-rich lower Eagle Ford Formation, which essentially is the source rock, and carbonate-rich upper Eagle Ford formation. Excursions in the carbon stable isotope data correlated with variations in XRF data may provide insight into ocean anoxic events which may be used to constrain time boundaries. Using a combination of the above mentioned methods, the study will provide detailed understanding of the depositional rock record and their subsequent diagenetic alterations, which may be critical in determining the “sweet spots” in the study area and ultimately placing wells in the right location.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016