PRODUCED WATER GEOCHEMISTRY OF THE ORDOVICIAN UTICA SHALE AND ADJACENT FORMATIONS IN THE APPALACHIAN BASIN
Our goal is to determine baseline geochemical brine compositions of the Utica and adjacent formations using produced water samples from mature wells to fingerprint different reservoirs and help identify fluid movement between reservoirs during hydraulic fracturing. Utica and conventional reservoir brine samples from western Pennsylvania and eastern Ohio were sampled and analyzed for major, trace, and isotopic compositions. Utica samples have calculated TDS concentrations of ~200,000 mg/L, within the range of Marcellus shale salinities yet orders of magnitude higher than previously analyzed Utica flowback waters. This supports the assumption that Utica produced waters from mature wells, like those from the Marcellus, represent natural formation water. The δ18O and δD isotopic results reveal brines that have remained isolated from meteoric surface waters yet are similar to other deep conventional reservoirs and may not be useful for uniquely identifying the brines. Strontium concentrations (> 5,000 mg/L), however, are higher in the Utica than nearly all other sampled formations in the basin, and plot in a restricted region of a 87Sr/86Sr vs 1/Sr mixing diagram, suggesting that Sr isotopes may be useful for fingerprinting Utica brines.