--> Lithium Isotopic Compositions of Devonian Brines and Marcellus Shale Rocks of the Central and Northeastern Appalachian Basin

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Lithium Isotopic Compositions of Devonian Brines and Marcellus Shale Rocks of the Central and Northeastern Appalachian Basin

Abstract

Saline waters co-produced from both unconventional shale gas and conventional sandstone wells provide a unique opportunity to investigate sources of the formation brines and water rock interactions. In this study, multi-collector ICPMS was used to measure lithium isotope ratios (δ7Li) in water and whole rock samples from hydraulically fractured wells in the Marcellus shale (Middle Devonian), and water from conventional wells producing from Upper Devonian sandstones. The distribution of lithium concentration in different minerals was determined using sequential extraction techniques. The results show that Li is primarily associated with silicate minerals. Structurally bound Li accounted for 75–91% by weight whereas exchangeable Li is less than 2%. In the Marcellus Shale, the main sink for Li is uptake by clays whereas carbonate cement contains negligible amounts of Li (< 2%). δ7Li values of shale whole rocks in both Greene Co., Pennsylvania and Tioga Co., New York ranged from −2.3‰ to +4.3‰, similar to values reported for shales in the literature. Time-series samples of Marcellus Shale water in southwestern Pennsylvania showed that both Li concentrations and δ7Li increased over the first 45 days of production then remained constant ([Li]=98±5 mg/L; δ7Li=10.3±0.2‰, 2SD) for 27 months of continued sampling. The initial trend can be explained by mixing of isotopically light Li in the injected fluid with formation brine containing high concentrations of isotopically heavier Li. The U. Devonian sandstone brines (Greene Co.) have δ7Li values in a narrow range (+13.7‰ to +15.4‰) and are distinct from Marcellus Shale produced waters from the same region (+7.63‰ to +10.5‰). This suggests minimal present migration of brine from Marcellus brines and deeper units upward into the overlying U. Devonian sandstones. Marcellus brines from Tioga Co., north-central Pennsylvania, contain more Li (208–233 mg/L) and isotopically heavier Li (14.3‰ to 15.0‰) than those in southwestern Pennsylvania. Our preliminary data for Marcellus Shale produced water suggest regional differences in the Li concentration and isotopic ratios, possibly reflecting differences in sources, the original water trapped in the shale fractures, diagenetic processes, thermal histories, or paleo-fluid flow pathways.