Although regional hydrogeologic data and numerical models suggest that ground water flows downward through the evaporite-confining system in the Palo Duro Basin, new evidence of chemical and isotopic composition of brine from a carbonate bed in the Permian San Andres Formation suggests that post-Permian ground-water movement within the evaporite section has been negligible. Similarity between δD and δ18O of brine in San Andres carbonate rock and δD and δ18O of Permian fluid inclusions in halite beds implies that brine in the carbonate rock is connate and originated as Permian evaporatively concentrated sea water. Extensive rock-water reactions that account for chemical and isotopic composition of the brine by circulating meteoric ground water seem more complex and less substantiated than does diagenetic change of connate Permian brine. The conflict between hydrogeologic and chemical interpretations is reconciled if (1) there has not been enough time for flow of meteoric ground water to flush connate brine from the carbonate bed since a significant cross-formational gradient in hydraulic head developed, (2) present cross-formational flow of ground water is unevenly distributed between fractured and unfractured areas, and (3) composition of brine sampled at the test wells differs from that of brine in fractured zones that have not been sampled.

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