Systematic petrological studies were performed at 10-m intervals along the 2700-m-deep Ch-A well. Results show mineralogical variations that define four zones which, in turn, represent different thermal zones. The shallowest zone (Zone 1) is characterized by the presence of chalcedony + zeolites + amorphous silica + saponite + montmorillonite + minor amounts of pyrite and calcite; Zone 2 by chlorite + quartz + smectite + zeolites; Zone 3 by chlorite + quartz + calcite + epidote + abundant pyrite and hematite + mixed-layered illite-smectite + chalcopyrite; and Zone 4 by epidote + clinozoisite + gypsum + sericite + mixed-layered chlorite-illite-smectite + anyhdrite. Fluid inclusion analyses performed at 100-m intervals indicate that a low-to-moderate salinity fluid with ice-melting temperatures of -0.7 to -2.2 degrees C was involved in the hydrothermal alteration of the rocks. At shallow depths, positive values of +1.6 degrees C were found,which probably indicate an increase in volatile components. Minimum homogenization temperatures gradually increased with depth. They range from 110 degrees C at very shallow depths (153m) to 244 degrees C at total depth (2700m); however, peak or maximum temperatures of 265-286 degrees C are found at various depths between 1400 and 2500m. Bottom-hole fluid inclusion temperatures agree well with static temperatures derived from the Horner (1951) and the Ascencio el al. (1994) methods. Comparisons at other depths show that, in general, Horner temperatures are the lowest and that fluid inclusion temperatures are the highest, except at about 1200m depth where the Ascencio et al. (1994) method gives the largest values. It is believed that well CBA encountered a mineral paragenesis that does not correspond with present thermodynamic conditions of the reservoir and that the geothermal system has undergone natural cooling.