Geochemical and isotopic fluid data from 10 wells completed in the Baca reservoir indicate the presence of both a normal enthalpy, lower-chloride fluid and an excess enthalpy, higher-chloride fluid. This chemical difference, along with enrichment of deuterium and oxygen 18 in the excess enthalpy fluid, can be explained by adiabatic cooling or boiling at 170 ¿C during upward convection along the central fault system in the reservoir followed by conductive reheating during downward movement. Chemical modeling using the EQ3NR computer code indicates chemical stability with the mineral assemblage quartz, albite, K-mica, epidote, chlorite, calcite, anhydrite, and pyrite, which is in agreement with observed minerals in well cuttings. This correlation, along with chemical geothermometers, implies a constant temperature range of 260¿--320¿C during evolution of the reservoir.