GEOLOGICAL, GEOCHEMICAL, AND HYDROLOGICAL CHARACTERIZATION OF CAMBRIAN-MISSISSIPPIAN GAS SHALES IN THE BLACK WARRIOR BASIN, ALABAMA

The Black Warrior Basin (BWB) in west-central Alabama contains Cambrian through Mississippian shales that may potentially produce up to 800 trillion cubic feet of natural gas. To better understand the potential environmental impacts and production potential of these shales, we characterized their inorganic and organic geochemistry, mineralogy, thermal and hydrologic evolution, and hydrologic properties.

X-ray fluorescence as well as X-ray diffraction were used to distinguish the variations in gas shale mineralogy and quantify the concentration of trace elements with potential to impact potable groundwater, should the mixing of brine fluids and groundwater occur. The results show that these shales contained varying amounts of quartz, calcite, sulfide minerals (e.g., pyrite and arsenopyrite), and clay minerals. Elevated concentrations of certain trace elements (e.g., As and Pb) are found in all but the Conasauga shale, which is dominated by carbonate minerals (up to 50% by weight). The Neal (Floyd) Shale has the highest sulfide mineral and As content. Trace metals often concentrate in fine-grained sulfide minerals which often serve as the major sinks for toxic metals such as As and Pb under reducing environments.

Geochemical characteristics of gas chromatograms and fragementograms of shale samples indicate different sources and evolution history of organic matter in each gas-shale. Despite the significant differences in overall gas chromatograph profiles, selected biomarkers in three shales have important geochemical similarities. Geophysical gamma logs were used to correlate various hydro-geologic units in the basin. A three dimensional hydro-stratigraphic framework of the BWB was reconstructed; from this, several two-dimensional transects were modeled for thermal and hydrologic evolution. Major over-pressurization within the BWB occurred during the deposition of the thick Pottsville Formation (Pennsylvanian). During this time the majority of the Neal (Floyd) and Chattanooga Shale fell into the oil window. The gas window in these shale units were not reached until the deposition of the Gordo Formation (Cretaceous). Permeability measurements, analysis of accessible porosity, and imaging of connected pore structures are underway to investigate the hydrologic properties of these shales.