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, hydraulic fracturing processes may enhance pyrite oxidation, and trace element release, from the shale to water. First, the introduction of oxygenated fracturing fluid to the shale source rock may oxidize the pyrite in situ, allowing trace elements to be released into flowback water. Flowback water is typically stored in retention basins, which can leak, allowing these toxic trace elements to enter water supplies. Second, shale rock cuttings from drilling are also exposed to oxygen at the surface, producing acid drainage and allowing trace element release. Many trace elements are toxic and are regulated by the U.S. EPA; therefore, examining the mineral sources and potential mobilization mechanisms is important for protecting water supplies in areas impacted by hydraulic fracturing.

In this study, pyrites and the shale matrix of gas-producing shales were evaluated for trace element content using laser ablation inductively-coupled mass spectrometry (LA ICP-MS). Four thin sections from four cores in the Marcellus Formation, the most expansive shale-gas play in the U.S., were analyzed by LA ICP-MS. NIST 612 glass was used as the external standard, and the USGS MASS-1 and BIR-1G were used as check standards. Results show elevated concentrations of some trace elements, including As and Cd, in pyrites compared to the shale matrix. In contrast, Ba, Cr, Ga, Mo, Mn, V, Th, and W are more abundant in the shale matrix than in the pyrite. Future research will include analyzing other phases for trace elements.