COUPLING LEAD ISOTOPE RATIOS AND ELEMENT CONCENTRATIONS TO TRACK PROCESSES INVOLVED IN RESOURCE EXTRACTION FROM THE MARCELLUS SHALE

A study on the coupled use of element concentrations and lead isotope ratios from leached drill cuttings was used to assess the potential for environmental impacts from unconventional natural gas resource extraction from the Marcellus shale. The use of dry drilling techniques allowed fresh, unaltered cuttings from 600 meters of the middle Devonian stratigraphic section from Tioga County, NY to be used for the leaching experiments. The leaching experiments included 24 hour deionized water (DI) and dilute hydrochloric acid (HCl) leaches on unoxidized cuttings, as well as a duplicate set of experiments on cuttings that undergone accelerated oxidization. Multi-element concentrations and lead isotope ratio measurements were determined through use of magnetic sector ICP-MS. The use of the HCl leach mimicked acid addition during hydraulic fracturing preparation processes, and when contrasted with the DI leach, was needed to partition the Pb, Cu, Al, Fe and REEs into a soluble form. In turn the accelerated oxidation, which mimics long term weathering processes, further increased the concentration of many elements of interest into soluble forms. Pb concentrations were higher in the portion of the stratigraphic section above, rather than within, the Marcellus shale. In contrast, major changes in lead isotope ratios are coupled with elevated ²³⁸U, V, Sb, Ni and Mo concentrations in the lower portion of the Marcellus shale. Microprobe characterization suggests iron sulfides occurred in framboidal as well as cubic forms within the Marcellus shale. Metals within the different types of iron sulfide lattices and adsorbed to organic matter likely controlled solubility of Pb and related species as supported by the lead isotope ratio work. Extensive water/rock interaction does not seem to have taken place in the Marcellus shale at this location following deposition. Rather the in-situ radioactive decay of uranium and thorium resulted in corresponding changes in Pb isotope ratios which are well preserved in the Marcellus shale portion of the stratigraphic section. Coupling differences in metal concentrations and lead isotope ratios can provide a fingerprint for characterizing sources of materials during and post unconventional natural gas extraction processes as metals partition between solid and aqueous phases.