THERMAL AND PETROLOGIC STUDY OF THE CH-A WELL FROM THE CHIPILAPA-AHUACHAPAN GEOTHERMAL AREA, EL SALVADOR

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 chalc edony + zeolites + amorphous silica + saponite + montmorillonite + min or amounts of pyrite and calcite; Zone 2 by chlorite + quartz + smecti te + zeolites; Zone 3 by chlorite + quartz + calcite + epidote + abund ant pyrite and hematite + mixed-layered illite-smectite + chalcopyrite ; and Zone 4 by epidote + clinozoisite + gypsum + sericite + mixed-lay ered chlorite-illite-smectite + anyhdrite. Fluid inclusion analyses pe rformed at 100-m intervals indicate that a low-to-moderate salinity fl uid with ice-melting temperatures of -0.7 to -2.2 degrees C was involv ed in the hydrothermal alteration of the rocks. At shallow depths, pos itive values of +1.6 degrees C were found,which probably indicate an i ncrease in volatile components. Minimum homogenization temperatures gr adually increased with depth. They range from 110 degrees C at very sh allow 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 temperatur es agree well with static temperatures derived from the Horner (1951) and the Ascencio el al. (1994) methods. Comparisons at other depths sh ow that, in general, Horner temperatures are the lowest and that fluid inclusion temperatures are the highest, except at about 1200m depth w here the Ascencio et al. (1994) method gives the largest values. It is believed that well CBA encountered a mineral paragenesis that does no t correspond with present thermodynamic conditions of the reservoir an d that the geothermal system has undergone natural cooling. (C) 1997 C NR. Published by Elsevier Science Ltd.