Formation waters from producing reservoirs in Paleozoic carbonates have been studied to determine the origin of brines on the eastern Central Basin platform in west Texas. Chemical and isotopic analyses of these waters indicate mixing of brines of quite different origins in the deep subsurface of the Permian basin. Formation waters from the middle Permian San Andres Formation at 1430 m (4700 ft) and from Devonian limestones at 3200 m (10,500 ft) have salinities of 26-59 g/L and delta D-delta(18)O values in the same range as modern precipitation and groundwater in the near-surface Ogallala aquifer, Na-Cl-Br concentrations and molar ratios show that the salinity of these waters was largely acquired through halite dissolution. Formation waters from Pennsylvanian and Lower Permian shelf limestones at 2600-3000 m (8500-9800 ft) are more saline (70-215 g/L) and apparently represent a mixture of two different fluids. One end member was highly saline, derived from seawater evaporated well beyond halite saturation, and the other end member was a moderately saline meteoric water similar to the San Andres and Devonian formation waters. Halite beds occur only in Upper Permian (upper Guadalupian and Ochoan) strata in this area; hence, extremely saline evaporated seawater apparently descended into the Paleozoic carbonates during halite deposition in the Late Permian, mixing with and displacing marine formation waters by buoyancy-driven convective flow. These modified evaporitic brines were the dominant fluids in the Paleozoic carbonates until the late Tertiary, when meteoric water began to flow into deeper Paleozoic strata from outcrops and near-surface aquifers in southeastern New Mexico in association with tectonic uplift that began at 5-10 Ma. The meteoric water dissolved halite and anhydrite from Permian evaporites near the basin margin and moved eastward along the regional hydraulic gradient, mixing with and displacing the modified evaporitic brine in deeper hydrogeologic systems. These late Cenozoic meteoric fluids probably are responsible for widespread biodegradation of oil in the San Andres/Grayburg interval. The results of this study indicate that meteoric waters can migrate large distances and displace saline waters deep in a basin that has numerous oil reservoirs that have solution-gas drive, Understanding the history of formation waters can assist in exploration and production through improved (1) interpretation of reservoir-rock diagenesis, (2) prediction of oil biodegradation and displacement, (3) understanding of subsurface water pressures, and (4) interpretation of hydrocarbon saturations from resistivity logs.