LITHIUM CONCENTRATIONS AND ISOTOPE RATIOS IN PRODUCED WATER FROM THE MARCELLUS FORMATION, PENNSYLVANIA, USA

Water from shale is not typically available for chemical analysis because formation water that accompanies conventional oil and gas production is from sandstone or carbonate reservoirs. However, hydraulic fracturing of shale to stimulate natural gas, natural gas liquids, and oil production also liberates formation waters, and these waters are co-produced at the well head with the gas and/or liquid products. Produced water from shale formations represents a new opportunity for understanding water-rock interaction in sedimentary rocks. We used quadrupole ICP-MS and multi-collector ICP-MS to determine concentration and isotope ratio of lithium, respectively, in produced waters from hydraulically fractured shales of the Middle Devonian Marcellus Formation from four counties in Pennsylvania spanning 375 km². In comparison with existing data from conventional oil and gas reservoir formation waters, the Marcellus produced waters contain elevated Li/Cl*10,000 (~10 to 15 vs. <1 to ~12). Other ratios (Li/Mg, Br/Cl) are also higher than most formation waters and geothermal fluids. In published analyses of formation waters, δ⁷Li varies inversely with concentration. The d⁷Li measurements of the shale fluids analyzed for this study (~6.5 to 11‰) are much lighter than average seawater (~31‰) and cluster at the lowest δ⁷Li and highest Li concentrations of formation waters published so far. These geochemical characteristics of Marcellus produced waters suggest strong influence from clay minerals and provide a distinctive and potentially useful diagnostic tracer for contamination investigations.