The Na/K molar ratio of the evaporated injection brine is fairly similar to that of the fluids produced by wells in the CP-I north-western area. Thus, the mixing of waters at depth could maintain the Na/K ratio, and therefore the temperature estimated from the K–Na feldspars exchange reaction would be not affected. The logged temperature values are between 240 and 270 °C for the shallower aquifer and around 260 °C for the deeper aquifer. The temperatures estimated by Na/K geothermometer (average temperature forαreservoir is 264 °C and forβreservoir 260 °C) compare well with the logged values indicating that re-equilibrium occurs in the system. In order to explain the thermal and productive evolution of the area,Rodríguez et al. (2000) pointed out that recharge in CP-I area should occur at depth. However, based on the results of this study, we are proposing that separate upflows of thermal fluids should exist for each reservoir, with both from the same source but flowing through different pathways as was discussed byPortugal et al. (2005b)for the shallower reservoir in CP-I area