Origin and Evolution of Deep-Basin Brines, Palo Duro Basin, Texas

Porous and permeable strata of the Deep-Basin Brine aquifer underlie bedded evaporites that are being considered as repositories for high-level nuclear waste isolation in the Palo Duro Basin, Texas Panhandle. Formation-water samples collected from four U.S. Department of Energy test wells and from two wells drilled by independent oil and gas companies in the basin were analyzed for chemical and isotopic compositions to characterize the geochemical environment, determine the origin and compositional evolution of the water, and augment previous results of hydrologic investigations of ground-water flow directions and flow times.Formation waters are sodium chloride brines that contain 140 to 290 g/L total dissolved solids, primarily as a result of contact with halite. Concentrations of major and minor ions appear to be controlled by equilibrium with calcite, dolomite, gypsum or anhydrite, celestite, low albite, microcline, and sodium smectite. High sodium activities drive ion exchange reactions, which, coupled with maintenance of chemical equilibrium, elevate the concentrations of divalent cations and lower the concentrations of dissolved sulfate and carbonate species. Brines from wells in the central and eastern parts of the basin are in or near oxygen isotopic equilibrium with calcite at subsurface temperatures, whereas brines from the western and northwestern margins of the basin are isotopically unequilibrated with calcite host rock.Evolution of deep-basin brines is interpreted from chemical and isotopic compositions, and interpretations are integrated with results of previous geologic and hydrologic investigations. Brines from the western and northern Palo Duro Basin acquired salinity by dissolving halite along a lateral flow path from recharge at outcrop. Brines from the central and eastern Palo Duro Basin originated as either evaporatively concentrated seawater or recharge fluid that achieved trace element and isotopic equilibrium with host rock because of long flow paths and residence times. Regional chemical and isotopic compositional variations confirm the results of hydrologic modeling. Brines in the western and northern parts of the basin are recharged from the west and southwest, where siliciclastic rocks predominate in the recharge zone. Brines in the central and eastern parts of the basin are recharged from the south and southwest, where carbonate rocks predominate. Although hydrologic models indicate a potential for significant cross-formational flow through overlying evaporite strata into the deep-basin ground-water system, no unequivocal evidence of volumetrically important mass transfer is indicated by the chemical or isotopic compositions of the samples analyzed.