The Cerro Prieto IV (CP IV) reservoir, located in the northeastern part of the Cerro Prieto (Mexico) geothermal field, was studied in order to define its pre-exploitation conditions and initial (2000-2005) response to exploitation. Bottomhole thermodynamic conditions were estimated by modeling heat and fluid flows using the WELLSIM program and well production data. Produced fluid chemical and isotopic data were also analyzed to investigate characteristic patterns of behavior over time, which were then compared against simulation results to obtain a conceptual model of the CP IV reservoir. According to the proposed model, two zones in the reservoir - separated by Fault H and producing fluids of different characteristics - were identified under pre-exploitation conditions. Wells in the area to the east-southeast (south block) produce very high-enthalpy fluids (>= 2000 kJ/kg), with very low chloride (<= 7000 mg/kg) and high CO(2) (> 6 parts per thousand molar) and delta D (<-94 parts per thousand). In contrast, wells toward the west-northwest (north block) show moderate-enthalpy fluids (1400-1800 kJ/kg). with high chloride (similar to 12,000 mg/kg) and relatively low CO(2) (< 6 parts per thousand molar) and delta D (<-94 parts per thousand). Dilution caused by cooler water entry, boiling due to steam gain, both occurring in the north block, and steam condensation in the south block were identified as the three main reservoir processes associated with exploitation. Also, it was found that the dynamics of the CP IV reservoir is controlled by the Fault H system. (C) 2011 Elsevier Ltd. All rights reserved.