GEOCHEMISTRY OF ORGANIC RICH DEVONIAN SHALES

Gamma ray (GR) spectroscopy is commonly used to map the organic-rich facies of hydrocarbon source rocks and a constant or basin-wide correlation between GR response and total organic carbon (TOC) content is often assumed. However, in the economically significant Devonian shales of the Appalachian Basin there is notable variability in the GR response, while TOC concentrations remain fairly consistent. GR response, which is mainly due to a decrease in uranium, is appreciably reduced in the Dunkirk, Pipe Creek, Rhinestreet and Middlesex shales, compared to that in the Marcellus shale, while TOC concentrations remain fairly consistent. In this research we investigate the influence of major, minor and trace element geochemistry on the GR response of these Devonian shales. We hypothesize that GR response may be influenced by changes in ionic abundances, particularly carbonate and phosphate, which affected the solubility, sorption, and reduction of uranium. We test this hypothesis by systematically investigating the association of uranium with mineral and organic fractions of Devonian black shale outcroppings and core samples from the western and northern edge of the Appalachian Basin. X-ray Absorption Spectroscopy (XAS) is used to identify the valency of uranium in shales with the greatest GR response. These results should be valuable in interpreting GR response in the Appalachian Basin and in other black shale hydrocarbon deposits.