Characterizing the Geological and Geochemical Properties of Selected Gas Shales and their Thermal Maturation in the Black Warrior Basin, Alabama

Abstract

This study will focus on selected gas shale’s, geological and geochemical properties in Alabama’s Black Warrior Basin, which contains Cambrian through Mississippian shales; these gas-shales that may potentially produce up to 800 trillion cubic feet of natural gas. This study was performed from a multidisciplinary standpoint where several important aspects of gas-shale production were examined where both industrial and environmental concerns of gas-shale were addressed. Environmental concerns were restricted to aspects of gas-shale production that could potentially contaminate groundwater. Considering industry concerns, special attention was paid to the hydrocarbon development in each of the gas-shales studied. To do this, several techniques were utilized to (1) characterize the variations in gas-shale mineralogy’s, (2) quantify the concentration of trace elements (e.g., those with potential impacts to drinking water), (3) characterize and correlate key organic compounds (i.e., biomarkers) extracted from shales, (4) model the thermal history and hydrodynamic evolution of the basin, and (5) understand how new regulations involving hydraulic fracturing may potentially affect the industrial practices of protecting groundwater supplies. X-ray diffraction (XRD) and X-ray fluorescence (XRF) techniques were used to characterize the variations in gas shale mineralogy and quantify the concentration of trace elements, especially those with potential to impact potable groundwater if mixing of brine fluids and groundwater occur. The XRD results show that these shales contained varying amounts of quartz, calcite, and sulfide minerals (e.g., pyrite and arsenopyrite). Elevated concentrations of certain trace elements such as arsenic (As) and lead (Pb) are found in all but the Cambrian Conasauga Shale, which is dominated by carbonate minerals (up to 50% by weight). The Neal (Floyd) Shale has the highest sulfide mineral and As contents. Trace metals tend to concentrate in fine-grained sulfide minerals, which commonly serve as the major sinks for toxic metals such as As and Pb under reducing environments. These particular toxic metals are currently regulated by groundwater regulations in Illinois, Colorado, and Pennsylvania, during gas-shale production. Similarities in gas fragmentographs of all three biomarkers associated with m/z 191, 217, and 218 suggest a common source of organic carbon for the Devonian Chattanooga Shale and Cambrian Conasauga Shale. By contrast, significantly different biomarker signatures of the Mississippian Neal (Floyd) Shale indicate that organic matter in this younger unit is likely derived from a different source. Geophysical logs (gamma logs) were used to correlate hydro-geologic units in the basin. A three dimensional hydro-stratigraphic framework of the Black Warrior Basin was reconstructed; utilizing this hydro-stratigraphic framework, a two-dimensional transect across the basin was modeled for thermal and hydrologic evolution. The modeling results indicate that major over-pressurization within the Black Warrior Basin occurred during the rapid deposition of the thick Pottsville Formation (Pennsylvanian). It was during Pennsylvanian that the majority of the Neal (Floyd) and Chattanooga shales reached the oil window; the gas window in these units was not reached until the erosion of the Upper Pottsville Formation during Late Pennsylvanian.