Abstract
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.
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