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Geochemical and Petrophysical Characterization of the Mississippian Limestone and Barnett Formation Along the Eastern Margin of the Central Basin Platform, Permian Basin, U.S.A.

Abstract

While mudrocks of the Barnett Formation have been studied extensively in the Ft. Worth Basin, Mississippian stratigraphy in the Permian Basin has not received the same attention. Many previous Barnett studies in the Permian Basin rely on wireline logs, comparing log signatures to core and outcrop descriptions from adjacent basins for characterization. These studies divide Mississippian-aged rocks of the Permian Basin into two units: basal, shallow water carbonates (Mississippian Limestone) and overlying siliciclastic mudrocks (Barnett Formation). The Mississippian Limestone is interpreted to have been deposited on a carbonate platform to the north and is overlain by the Barnett Formation, that was deposited as deep-water hemipelagic sedimentation in prevailing suboxic to anoxic conditions. Recent production from the Barnett in south-central Andrews County, Texas, indicates Mississippian strata as a promising target for unconventional hydrocarbon production. However, relatively little has been published about the regional variability in source rock quality and petrophysics of Mississippian stratigraphy on the Central Basin Platform (CBP) of the Permian Basin. This study aims to characterize the temporal and spatial variation in mineralogy and organic matter of the Mississippian succession along the eastern margin of the CBP. Organic and inorganic geochemical methods (XRF, XRD, LECO TOC, and programmed pyrolysis) are incorporated with porosity measurements from well cuttings to test the hypotheses that, 1) organic matter is most highly concentrated in the Lower Barnett mudrocks of the CBP, and 2) matrix porosity in the Mississippian Limestone and Barnett is inversely related to the relative abundance of clay minerals. Preliminary results suggest the Lower Barnett increases in source rock quality to the south, along the eastern margin of the CBP, while the Mississippian Limestone is organic matter poor across the CBP. Additionally, redox sensitive elements (Cu, Ni, Mo, and U) display a positive correlation with TOC in the Lower Barnett, supporting previous models of deposition under prevailing suboxic to anoxic conditions. Pore volume and pore size distribution measurements from nuclear magnetic resonance (NMR) and low-pressure gas adsorption techniques will be compared with XRD and XRF measurements to examine the relationship between matrix storage potential and mineralogy in the Mississippian Limestone and Barnett Formation.