DETERMINING FACIES AND ELEMENTS OF PRODUCTIVITY IN GAS SHALE PLAYS: AN EXAMPLE FROM THE DEVONIAN CHATTANOOGA SHALE IN ALABAMA AND TENNESSEE

Devonian to Mississippian shales of the Ouachita and Appalachian orogenic belts produce natural gas at isolated locations across the southern midcontinent region and southern Appalachians. The shales are known by different names in different geographic regions; Barnett in northern Texas, Woodford in southern Oklahoma, Fayetteville in Arkansas, and Chattanooga in Alabama and Tennessee. Although all these units were deposited in deep marine environments in the Paleozoic Iapetus (Proto-Atlantic) Ocean, there are certain lithologic and/or diagenetic properties that influence productivity in different regions today. These properties are likely related to specific mineralogical and geochemical compositions that enhance gas and oil generation and trapping. If certain rock properties are found within a specific lithofacies, a predictive tool can be developed to locate areas of higher productivity that represent areas where the coincidence of geologic factors may result in economically producible accumulations of oil and/or gas.

We are developing a regional three-dimensional facies model of the Devonian Chattanooga Shale in Alabama and Tennessee. Mineralogical and geochemical data from outcrops and subsurface sources will be used to determine total organic carbon (TOC), maturity, and geochemical parameters to identify source rock locations and reservoir facies. Fractures, essential for gas shale production, are examined to determine their physical characteristics (e.g., height and width), frequency, and density as related to host lithotype and orientation. The relationship of these features to major tectonic episodes will be determined. Burial history is also being evaluated by examining the thermal alteration of organic materials and microfossils, diagenetic changes in clays, and progression of silica diagenesis and induration.