Hydrogeology and Hydrochemical Facies of the San Andres Formation in Eastern New Mexico and the Texas Panhandle

Fluid pressure, permeability, rates and patterns of ground-water flow, and chemical and isotopic composition of ground water in carbonate rock in the lower part of the San Andres Formation (Permian) in eastern New Mexico and the Texas Panhandle were studied to characterize the origin and movement of brine within an evaporite-carbonate-shale confining system. Although hydraulic-head data and numerical models suggest that ground water flows downward through the evaporite confining system in the Palo Duro Basin of the Texas Panhandle, evidence derived from study of chemical and isotopic composition of brine from two test wells indicates that post-Permian groundwater movement in the unit 4 carbonate bed of the San Andres Formation has been negligible in the Palo Duro Basin. The conflict between the chemical interpretation and the hydrologic model predictions can be reconciled if (1) the amount of ground-water flow since a significant cross-formation gradient in hydraulic head developed during the Pleistocene has not been enough to flush connate brine from carbonate beds, (2) present ground-water flow is unevenly distributed between fractured and unfractured zones in the evaporite confining system, and (3) brine sampled at test wells differs from other waters in fractured zones not yet sampled. Hydraulic head of ground water in the San Andres Formation appears to be hydrodynamically controlled by land-surface topography and by production of hydrocarbons and formation water. A ground-water-basin divide inferred from a fold in the potentiometric surface corresponds to the Western Caprock Escarpment and indicates that San Andres ground water below the Southern High Plains is separated from the karstic aquifer in the San Andres Formation in the Pecos Plains. The separation is reflected in a marked difference in hydrochemical facies and salinity. San Andres ground water varies from Ca-HCO3 and Ca-SO4 hydrochemical types with salinities of less than 5,000 mg/L in the Pecos Plains of eastern New Mexico to Na-Cl and Ca-C1 brines with salinities of 336,000 to 384,000 mg/L and Na/CI mole ratios of as low as 0.2 beneath the Southern High Plains, east of the ground-water-basin divide. The groundwater-basin divide probably developed during the Pleistocene when the ancestral Pecos River excavated its valley. Similarity between δD (-15 ‰ to -26 ‰)and δ18O (+4 ‰ to 6.4 ‰) of San Andres ground water in the Palo Duro Basin and δD and δ18O of fluid inclusions in halite beds suggests that brine in the carbonate rock could have originated during the Permian as evaporatively concentrated seawater. Dolomitization, ion exchange, clay diagenesis, and loss of dissolved sulfate account for possible modifications of the hypothetical Permian connate brine. However, with a meteoric-water model for brine origin, extensive rock-water reactions, such as ion- and isotope-exchange and incongruent recrystallization of halite, are needed to derive the observed composition. Isotopic exchange between meteoric water and San Andres carbonate rocks cannot account for δ18O composition of San Andres Formation brine. The reactions needed in a meteoric-water model seem more complex and less documented than simple preservation of a modified-connate, Permian brine.