The origin of reservoir fluids in the geothermal field of Los Azufres, Mexico - isotopical and hydrological indications

The calculation of hydrological balance resulted in a potential, average an nual infiltration rate of 446 +/- 206 mm/m(2) for the Los Azufres geotherma l area, which corresponds to a total of 82 x 10(6) m(3) per a. Due to the h ighly fractured and faulted structure of the volcanic formations, a conside rable potential for the infiltration of recent meteoric water into deeper s ections of the volcanic formations can be assumed. Isotopic data indicate t he minor importance of recent meteoric water for the recharge of the geothe rmal reservoir. Very negative delta C-13 values can be explained by the inp ut of organic C from the surface, but the lack of C-14 in the deep fluids r eflects a pre-historic age for the infiltration event of fossil meteoric wa ter. The dilution of the meteoric water by C-14-free CO2 gas from a shallow magma chamber complicates the exact age determination of the infiltration event, which probably occurred during the Late Pleistocene or Early Holocen e glacial period. Strong water-rock interaction processes, such as sericiti zation/chloritization, caused the primary brine composition to be camouflag ed. A preliminary hydrological model of the reservoir can be postulated as follows: the fossil hydrodynamic system was characterized by the infiltrati on of meteoric water and mixing with andesitic and/or magmatic water. Stron g water-rock interaction processes in the main part of the production zone prove the existence of former active fluid circulation systems. Due to chan ges in pressure and temperature, the rising fluids get separated into liqui d and vapour phases at a depth of 1500 in. After cooling, the main portions of both phases remain within the convective reservoir cycle. Isotope analy ses of hot spring waters indicate the direct communication of the reservoir with the surface at some local outcrops. A recent reactivation of the hydr odynamic system is caused by the geothermal production, as indicated by the detection of lateral communication between some production and reinjection wells. (C) 2001 Elsevier Science Ltd. All rights reserved.