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Geochemical and lithium isotope tracking of dissolved solid sources in Permian Basin carbonate reservoir and overlying aquifer waters at an enhanced oil recovery site, northwest Texas, USA

Pfister, Samantha, Capo, Rosemary C., Stewart, Brian W., Macpherson, G.L., Phan, Thai T., Gardiner, James B., Diehl, J. Rodney, Lopano, Christina L., Hakala, J. Alexandra
Applied geochemistry 2017 v.87 pp. 122-135
Guadalupian epoch, Neogene period, Triassic period, aquifers, basins, geochemistry, groundwater, hydrochemistry, isotopes, lithium, mixing, oils, sandstone, shale, temporal variation, Europe, Middle East, Texas
Geochemistry and lithium isotope compositions (δ⁷Li) of Permian Basin produced water and potable groundwater from overlying aquifers at an enhanced oil recovery (EOR) site in Gaines County, northwest Texas, are used to evaluate the effects of brine-groundwater-rock interactions, identify sources of dissolved solids, and characterize fluid flow and mixing processes. δ⁷Li values (per mil deviations from the LSVEC standard ⁷Li/⁶Li ratio) for produced water from dolostones of the San Andres Formation ranged from +10.9 to +15.6‰ and fall within the range of formation waters from other sandstone/carbonate reservoir rocks in North America, Europe and the Middle East. These differ from produced waters from hydraulically fractured shales from the U.S. Appalachian Basin, including the Marcellus Shale, which tend to have lower δ⁷Li values and higher Li/Cl, possibly indicating greater interaction with a terrigenous component. The San Andres produced water chemistry and Li isotope ratios are consistent with Neogene meteoric water interacting with marine and continentally-derived evaporites (e.g., portions of the Guadalupian Salado Formation), as well as other terrestrial sources along the flow path.Groundwater from the Triassic Dockum Group-Santa Rosa aquifer (δ⁷Li range of +20.6 to +23.5‰) is isotopically distinct from waters from the overlying Ogallala Formation (+10.6 to +16.5‰) and the deeper San Andres Formation, indicative of hydrologic isolation from both meteoric recharge and from deeper brines in the field area. In addition to tracking groundwater-brine mixing and water-rock interaction, temporal changes in the δ⁷Li composition of deep groundwater in the study area has potential use in the early detection of upward or injection-induced brine migration, prior to its incursion into the sensitive overlying Ogallala aquifer.