WATER-ROCK INTERACTIONS IN THE DEEP RIVER BASIN, NORTH CAROLINA: A CANDIDATE BASIN FOR SHALE GAS DEVELOPMENT
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Abstract
As a result of the rapid expansion of shale gas development in the United States, the public has become increasingly concerned about the proper management of hydraulic fracturing wastewater also referred to as produced water. Adverse environmental and human health implications may occur should there be a release of untreated or inadequately treated produced water into the environment. Consequently, there is a need to be able to identify environmental signatures of produced water to understand the fate of unrecovered fluids and to delineate the source and extent of accidental releases of produced water into the environment. In a previously little-drilled basin, few geochemical data exist on natural formation waters. The objective of this study was to simulate potential geochemical signatures in formation water from a non-marine, Mesozoic rift basin in order to predict the environmental signature of produced water. A series of sequential leaching extractions were conducted to simulate the influence of mineralogy, grain size, lithofacies and lithofacies association on the potential water-rock interactions in the Mesozoic Deep River basin, North Carolina. A comprehensive mineralogical study was completed to characterize the mineralogy and grain size of the sedimentary deposits involved in the water-rock interactions. The sequential extractions examined (1) the possible sources of extractable elements such as exchangeable sites on clay minerals and/or carbonate minerals, and (2) the solubility and leaching potential of a suite of elements into the environment. These data may provide insight into the naturally-sourced components of produced water should hydraulic fracturing occur in the Deep River basin. The geochemical data from this study suggest that the mineralogy and degree of post-depositional alteration of a deposit such as the presence or absence of carbonate minerals and/or grain size influence the water-rock interactions in the Deep River basin. The average concentration of extractable boron released from all of the Deep River basin samples and from all steps of the sequential extractions was lower (0.3 µg/g) than the average concentration of extractable strontium and barium, 11.9 µg/g and 19.3 µg/g, respectively. Boron was extracted primarily from carbonate minerals (average 0.7 µg/g). Strontium and barium were both preferentially leached from exchangeable sites, average 25.2 µg/g and 51.8 µg/g, respectively, with lesser amounts of strontium leached from carbonate minerals, average 18.8 µg/g. Although strontium and barium were preferentially leached from exchangeable sites on clay minerals, the geochemical data show that grain size may play a more influential role compared to clay mineral abundance in determining the solubility of extractable strontium and barium. While clay mineral abundance does not strongly influence the water-rock interactions, the clay mineral assemblage, does influence the water-rock interactions in the Deep River basin. For example, deposits with kaolinite as the predominant clay mineral leached lower concentrations of ammonium acetate extractable strontium and barium (9.2 µg/g and 45.2 µg/g, respectively) compared to deposits with smectite as the predominant clay mineral, 35.9 µg/g and 64.9 µg/g, respectively. Lastly, the geochemical data indicate that carbonate mineral content is strongly positively correlated to the concentration of extractable strontium The geochemical data from this study suggest that should hydraulic fracturing occur in the Deep River basin, the produced water generated by fine-grained deposits such as mudstones and shales that are characterized by the highest abundance of carbonate minerals are likely to generate elevated concentrations of extractable strontium and barium. The geochemical data also suggest that compared to the marine deposits of the Marcellus Shale, the non-marine, lacustrine deposits of the Deep River basin will likely generate significantly lower concentrations of total dissolved solids.