Geochemistry of the Marcellus Shale; Metals mobility and solubilization
Malizia, Thomas R.
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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, such as the Marcellus shale, there is a 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 Upper Devonian shales: Dunkirk, Pipe Creek, Rhinestreet and Middlesex, compared to that in the Marcellus shale, while TOC concentrations remain consistent. This research investigates the influence of major, minor and trace element geochemistry on the GR response of these Devonian shales. It is hypothesized that GR response may be influenced by changes in ionic abundances, particularly carbonate and phosphate, which affected the solubility, sorption, and reduction of uranium. The hypothesis is tested by systematically investigating the association of uranium with mineral and organic fractions of Marcellus shale outcroppings and core samples from the western and northern edge of the Appalachian Basin. X-ray Absorption Spectroscopy (XAS) was used to attempt to identify the valency of uranium in the shale 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.