An interpretation of the depositional environment and facies of the Eagle Ford Shale from Karnes-Maverick County, Texas

Development of shale reservoirs can be problematic due to internal heterogeneity. Areas or zones of higher clay content (such as swelling clays), variable lithology, total organic content variations, and changes in porosity and permeability, all can affect hydrocarbon recovery. These low porosity and permeability reservoirs are now significant hydrocarbon producers so it is crucial to understand their architectural elements and reservoir properties in order to maximize hydrocarbon production.

Phase one of this study included 6 wells with whole core across the Upper Cretaceous Eagle Ford Shale in South and East Texas (from Karnes to Maverick County). These cores 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. Identification of gently inclined lamina and ripple cross laminations provide evidence of significant bedload transport in the form of floccule ripple migration, indicating a depositional environment above storm wave base.

Bioturbated marl and laminated marl facies were used to identify flooding surfaces. These flooding surfaces separate upward-coarsening facies from bioturbated and laminated marl facies leading to the identification of parasequences. From this, we built a detailed sequence stratigraphic framework of the Eagle Ford Shale using the identification and correlation of parasequences and parasequences sets. From this work, a basin wide cross-section correlating these 8 facies was constructed using private and public data.

This new model is being used to target and geo-steer our wells into the best quality rock, thereby reducing drill time, increasing completions effectiveness, and maximizing production.