TRACE ELEMENT ANALYSIS OF PYRITES IN GAS SHALES BY LASER ABLATION ICP-MS: IMPLICATIONS FOR MOBILITY

Trace elements contained in pyrite-bearing shale have the potential to be mobilized when the shale is exposed to oxygen. Although this occurs naturally, the hydraulic fracturing life cycle may enhance pyrite oxidation, and trace element release, from the shale to the water. The trace element content of sedimentary pyrite has been examined using laser ablation inductively-coupled plasma mass spectrometry (LA ICP-MS); however, there are limited studies which use these data to evaluate the potential mobilization mechanisms as a result of the hydraulic fracturing of natural gas.

In this study, pyrites of gas-producing shales were analyzed for trace element content using LA ICP-MS. Thin sections from four cores collected from the Marcellus Formation, the most expansive shale-gas play in the U.S., were analyzed for a total of 99 analyses. Prior to analysis, the thin sections were examined under reflected light with a petrographic microscope to determine pyrite grain location and size. NIST 612 glass was used as the external standard, USGS MASS-1 and BIR-1G were used as verification standards, and Fe was used as the internal standard. Beam size ranged from 10-60 µm. Results show elevated concentrations of As, Se, Ni, Sb, Co, and Mo in pyrite in comparison to the bulk rock data, which were collected using instrumental neutron activation analysis (INAA). Because several of these trace elements are toxic to humans and are regulated in natural waters by the U.S. EPA, examining the mineral sources and potential mobilization mechanisms is important for protecting water supplies in areas exposed to hydraulic fracturing. Trace elements in pyrite may be released into flowback water due to the introduction of oxygen via the fracturing fluid. Flowback water has the potential to leak from improper well casings; it is also often stored in retention basins which have the potential to leak. Shale rock cuttings produced during extraction and disposed on the surface often contain pyrite, which when exposed to oxygen, have the potential to generate acid rock drainage and allow additional trace element mobilization. Future research will include analyzing additional thin sections from the Marcellus and other gas-producing shales for trace element concentrations, and evaluating other mineral phases in the shales for trace elements.