Facies and stratigraphic framework of the Eagle Ford Shale in South Texas

The Upper Cretaceous Eagle Ford Shale, in South and East Texas, consists of organic matter-rich fossiliferous marine shale. It is one of the most actively drilled targets for oil and gas in the United States, due to new technologies in drilling and completions. These low porosity and permeability reservoirs are now significant hydrocarbon producers, and therefore, it is crucial to understand the architectural elements and reservoir properties to maximize hydrocarbon production. Zones of higher clay content, variable lithology, total organic content variations, and changes in porosity and permeability can affect hydrocarbon recovery. This study focuses on building a detailed stratigraphic framework of the Eagle Ford Shale. Six wells with whole core through the Eagle Ford Formation were analyzed and interpreted to determine lithology, sedimentary structures, and parasequences. Eight separate facies along with their depositional environment were interpreted to have been deposited above storm wave base along the inner and outer shelf in a moderate energy environment episodically interrupted by higher energy events, as opposed to a deeper-water setting. Gently inclined lamina and ripple cross laminations provide evidence of significant bedload transport, probably as floccule ripples. Bioturbated horizons mark flooding surfaces that cap upward-coarsening facies successions, interpreted as parasequences. These surfaces, along with bentonites seen in core were then correlated to the corresponding wireline logs. These surfaces along with other distinct gamma ray markers were then correlated across the South Texas region using a dataset of 735 gamma ray logs. Several units show onlap and truncation that were used to document onlap and thinning adjacent to structural highs. The upper Eagle Ford Shale shows onlap and thinning of units towards the San Marcos Arch as opposed to the Maverick Basin region where the upper Eagle Ford Shale thickens significantly. The lower Eagle Ford Shale remains relatively constant throughout the South Texas region, showing only minimal areas of thickened sections. Thus, indicating deposition of this lower unit prior to subsidence of the Maverick Basin.