CHEMOSTRATIGRAPHY OF THE MISSISSIPPIAN LOWER BARNETT SHALE FROM THE PERMIAN BASIN, TEXAS: INSIGHTS INTO THE REDOX STRUCTURE OF THE TOBOSA BASIN

The Barnett Shale is a well-known unconventional shale reservoir in the USA with up to ~8% total organic carbon (TOC) content. The Mississippian System has been widely explored in regionally, but few studies of the Barnett Shale in the Permian Basin have been made publicly available. Recently, interest in the Barnett Shale in the Permian Basin has increased following the success of hydraulic fracturing activity in the basin over the past decade. The Barnett Shale was deposited in an outer ramp to basin setting in the Tobosa Basin of west Texas and southeastern New Mexico and consists of hemipelagic sediments and debris flows on the southern margin of Laurentia. The collision with Gondwana in the middle-late Mississippian subdivided the Tobosa Basin into the Midland and Delaware Basins with the uplift of the Central Basin Platform. Here we report on the sedimentology and geochemistry from the Lower Barnett interval (Osagean) from ~60 m (~195.5 ft) of core from Northeast Ector County, Texas.

Core observations were collected at > 2.5 cm resolution to distinguish skeletal components, sedimentary structures, and lithologic changes throughout the core. Six facies representing hemipelagic mudstone to bioclastic packstone debris flows were identified. The core can be divided into three subunits with the lower unit dominated by bioclastic wackestone-packstone, a middle unit dominated by skeletal wackestone-packstone, and an upper unit dominated by mudstone. The core was scanned at ~6 cm (0.2 ft) intervals with a Bruker Tracer 5G portable X-ray fluorescence (pXRF) spectrometer. Major and trace element concentrations, element ratios, and shale normalized enrichment factors were plotted stratigraphically to evaluate geochemical variation through the cored interval. An increase in TOC and redox sensitive trace metals, which suggest deposition in anoxic to euxinic waters, along with a decrease in grain size and bed thickness supports the overall deepening trend observed in the core. Combined the sedimentologic and geochemical data presented here provide valuable information for the Barnett Shale from the center of the Tobosa Basin that may be used to further understand the geochemical evolution along the southern margin of Laurentian carbonate platform.