Geochemical investigation of the basal Barnett Shale

A total of four cores of the Barnett Shale, provided by Devon Energy, were investigated for their geochemical characteristics. These four cores covered an area of approximately 100 square miles in the northeast region of the Fort Worth Basin. The basal Barnett Shale was the interval of interest for this study. The basal Barnett shale unconformably overlies Ordovician carbonates and is conformably overlain by less radioactive younger Barnett Shale beds. Total organic carbon was determined by coulometric titration and trace element concentrations were determined by XRF. From the geochemical data gathered, well-known proxies were used to infer depositional environments of the basal Barnett Shale. The inferred depositional environments were then used to build a depositional model of the basal Barnett Shale for the four cores. It was concluded that the basal Barnett Shale was deposited under anoxic bottom water conditions in all four cores, with occasional episodes of euxinia. The method of organic carbon sequestration was found to differ between the four cores. For the Bonds Ranch, Kathryn Hollingsworth and John W. Porter 3 organic carbon sequestration is interpreted to have been driven by increased productivity in the water-column, possibly being deposited under an upwelling zone. Whereas in the Ernest Smith core, bottom-water anoxia was interpreted to already have been present thus preserving any organic matter that was deposited to the seafloor. From this interpretation it was theorized that the Bonds Ranch, Kathryn Hollingsworth and John W. Porter 3 would be better drilling prospects than the Ernest Smith because of the increase in the three cores in biogenic silica from being deposited under or near an upwelling zone.