CHEMISTRY AND MINERALOGY OF DEVONIAN STRATA: A PRELIMINARY STUDY TO DETERMINE POSSIBLE BENEFICIAL USE OF WELL CUTTINGS

Well cuttings resulting from increased development of natural gas in the northern Appalachian Basin might form a potentially useful field additive that could replace or supplement agricultural limestone. The mineralogical composition of well cuttings, particularly the relative abundance of reactive pyrite, is a potential barrier to beneficial use. Possible mobile metal leachate from cuttings must also be addressed. In a preliminary study to assess methodologies and develop protocols, well cuttings and outcrop samples from Devonian strata in central New York State were analyzed using X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) in order to determine major and trace element composition, and mineral abundance. The USGS software package RockJock was used to quantify the modal mineral composition based on XRD data. Leachates from well cuttings were analyzed for mobile metals including lead, copper and zinc. The samples include well cuttings of upper Devonian units from the Matejka well located in Erin, NY. Field samples from New York include lower and middle Devonian shale and siltstone from the Tristates and Hamilton Groups, including intervals within the Marcellus Shale.

Major element and minor element geochemical data from XRF analyses indicate that the Devonian strata compare favorably to standard shale composites in the levels of potentially mobile and deleterious metals such as strontium, zinc, copper, and lead. Mineralogical abundances calculated from XRD patterns are consistent with point counts of polished shale chips examined using SEM, suggesting that mineral abundances can be readily calculated from XRD data using RockJock. Levels of copper, lead and zinc samples generally correlate positively with pyrite abundance. Marcellus interval samples contain 1-2% pyrite and 2-25% calcite, suggesting that sufficient buffering capacity exists to counter acidity produced by pyrite oxidation.