Sedimentary and Diagenetic Controls on Petroleum System Characteristics of the Upper Cretaceous Eagle Ford Group, South Texas

Early diagenetic carbonate cements can affect brittleness and total organic content in shale reservoirs. Predicting these effects could potentially improve recovery efficiency and field development costs, and decrease the environmental impact of developing the field. In this study, an X-ray fluorescence spectroscopic technique was used to test for correlations between primary depositional features, diagenetic carbonate cements, and organic content and fracture distributions in core samples from the Eagle Ford Group in McMullen County, Texas. Organic content varies significantly between diagenetic facies, with the least organic matter present in coarsely mineralized shales. This result is consistent with the hypothesis that diagenetic carbonate cementation that was early relative to compaction diluted primary organic matter. In contrast, total fracture length varies significantly between depositional facies, with the lowest total fracture length per length of core present in massive shales. Carbonate diagenesis therefore likely did not exert a significant control on the formation of the bedding-parallel fractures observed in this study; instead, laminated fabrics provided planes of weakness along which stress release fractures or hydrocarbon generation-induced fractures could develop. The suggested target reservoir facies for similar Eagle Ford wells is a finely to moderately mineralized laminated shale because of the likelihood of finding high organic content and horizontal fractures that would increase the effective rock volume in communication with primary hydraulically induced fractures.