Solute geothermometry of Cerro Prieto and Los Humeros geothermal fields, Mexico: considerations on chemical characteristics of thermal water

Abstract

A comprehensive study on the chemical considerations of thermal waters (springs and geothermal wells) on the performance of solute geothermometers in predicting the reservoir temperatures of Cerro Prieto and los Humeros geothermal fields of Mexico has been carried out. The reservoir temperatures from these thermal waters were calculated by applying all the available 28 solute geothermometers and the obtained temperatures were compared with the bottom hole temperatures (BHT) of the geothermal wells. Thermal waters of springs and geothermal wells of Cerro Prieto geothermal field (CPGF) are Cl type and majority of them are partially equilibrated. In the case of Los Humeros geothermal field (LHGF), spring waters are HCO3 type and all of them have indicated non-equilibrium conditions. Majority of the predicted reservoir temperatures by Na/K geothermometers from spring waters of CPGF and LHGF are within ±20 % differences compared to average BHTs, whereas estimated temperatures by all other solute geothermometers (non-Na/K) from spring waters of both the geothermal fields have shown wide differences (> ±20 %). In the case of geothermal well waters, majority of the reservoir temperatures estimated by only Na/K geothermometers from well waters of CPGF are within ±20 % differences and in the case of wells of LHGF, majority of the reservoir temperatures estimates by all solute geothermometers are having differences > ±20 %. The present study indicates the following: (1) only 3 geothermometers (out of 28) from spring waters of CPGF have predicted reservoir temperatures similar (statistically no significant differences) to the average of BHTs, although all springs are of Cl type; (2) different chemical characteristics of spring and well waters of CPGF and LHGF may be the result of the wide differences in the lithology of their reservoirs; (3) more number of solute geothermometers from non-equilibrated spring waters of CPGF and LHGF have predicted reliable reservoir temperatures than partially equilibrated waters; (4) In general, Na/K geothermometers have predicted more reliable reservoir temperatures from spring and well waters of both geothermal fields, when compared to other solute geothermometers; (5) the prediction of non-reliable reservoir temperatures by other than Na/K geothermometers may be the result of dilution/mixing of deep thermal waters with the meteoric surface waters; and (6) more number of geothermometers have predicted reliable reservoir temperatures from well waters in comparison to spring waters of both the studied geothermal fields.