MINERALOGY AND TRACE ELEMENT GEOCHEMISTRY OF GAS SHALES IN THE UNITED STATES: WATER/ROCK INTERACTIONS AND PRODUCED WATER IMPLICATIONS

The geochemical characteristics of produced water from hydraulically-fractured shale are in part controlled by the mineralogy and trace element chemistry of the gas shale material. We have compiled mineralogic and trace element data for nine hydrocarbon-producing shales in the U.S, including the Antrim, Bakken, Barnett, Eagle Ford, Haynesville, Marcellus, New Albany, Utica and Woodford. These data were used to gain insight into potential environmental impacts from the material and from potential water/rock interactions. To assess the distribution of trace elements in the gas shale material, we conducted statistical analyses on the combined shale dataset and between the shales and also compared concentrations to EPA screening limits for residential and industrial soils. The comparison revealed that for the trace elements examined in the combined dataset (As, Ba, Co, Cr, Mo, Ni, U, V, and Zn), only average As concentrations exceeded both residential and industrial soil screening limits. This does not imply that all gas shales have As concentrations that are of concern, but is does suggest that concentrations of As and other trace elements that may have adverse environmental impacts should be measured and managed appropriately. It is also important to recognize that although the means for trace element concentrations may fall below a regulatory standard, due to the natural variability in concentrations of trace elements in shales there are likely individual samples with concentrations that exceed standards. To assess the potential of the gas shale material to oxidize and produce acid rock drainage, we compared the ratio of pyrite to calcite in the shales. Results show that the Eagle Ford and Antrim formations generally contain excess calcite compared to pyrite while the Bakken, Barnett, Haynesville, Marcellus, and Woodford primarily plot in the pyrite excess field. The trace element distribution information combined with the mineralogic data can aid in the interpretation of observed trace element concentrations in produced water and also help prioritize potential constituents of concern. Compilations of the mineralogy and geochemistry of gas shales can be a valuable resource for managing real and perceived environmental problems associated with their exploitation and help to identify gaps in current data.