Developments of auxiliary chemical geothermometers (Na-Li, Na-Cs) applied to the Los Humeros and Acoculco high-temperature geothermal fields (México)
Résumé
The knowledge of the temperature of the deep geothermal fluids, rock permeability and water reservoir capacity are three key parameters for developing deep geothermal energy. Since 1965, one of the major applications of fluid geochemistry in the exploration of the potential geothermal reservoirs involves estimation of their temperature, using classical chemical, isotope and gas geothermometers on fluids collected from geothermal wells and thermal springs. The classical Na-K-Mg ternary diagram and main geothermometers such as Silica-quartz, Na-K, Na-K-Ca, K-Ca, and δ18OH2O-SO4, indicate that full chemical equilibrium is reached at about 290 ± 30°C for most of the geothermal waters discharged from the Los Humeros field. This temperature range is concordant with the presence of an upper liquid-dominated reservoir area, with neutral pH at 290-330°C, located in augite and hornblende andesites, or in deeper basalts, as suggested in numerous studies. Among the different Na-Li geothermometric relationships existing in the literature, only the Na-Li auxiliary geothermometer defined for North-Icelandic high-temperature basaltic geothermal dilute waters give concordant temperature values (320 ± 30°C). The Na-Cs auxiliary geothermometer also yields similar temperature values (300 ± 30°C). The classical Na-K-Mg ternary plot indicates that al the thermal waters discharged from the Los Humeros and Acoculco areas are immature, having not reached full chemical equilibrium with the host rocks. In addition, solute geothermometry cannot be applied to acidic waters. Despite their common meteoric origin indicated by the δD and δ18O isotopes, the Cl/Br mass ratios of these thermal waters are much lower (from 70 to 700) than those of the Los Humeros geothermal fluids (from 1200 to 1700). The Ca-HCO3 diagram shows that most of these thermal waters have interacted with calcium carbonates. They have high Ca, Mg and Sr concentrations compared with the Los Humeros geothermal waters, which are depleted in these elements. Their 86Sr/87Sr ratios, ranging from 0.706272 to 0.707262, suggest that they are interacting with marine carbonates formed during the Mesozoic period. Nevertheless, it appears that the Na/K and Na/Li ratios, for numerous thermal waters, are close to those of the Los Humeros geothermal fluids. Associated with δ7Li values ranging from 4.7 to 6.5‰ and relatively high B concentrations, and some escapes of deep reservoir hot gases which reach the surface, these ratios suggest that small fluxes of deep waters, despite their low-permeability environment, could be able to preserve, more or less, their original Na/K and Na/Li ratios (and to a lesser extent, their Na/Rb and Na/Cs ratios), resulting from the temperature of their deep reservoir, even after significant mixing with cold waters and chemical changes, during their ascent up to the surface. In such a context of low-permeability, these ratios may be useful tools for high-temperature geothermal exploration. Classical geothermometers such as Silica, K-Mg and K-Ca, based on fluid equilibration with chalcedony, muscovite, clinochlore, K-felspar and calcite, which can re-equilibrate relatively fast, as well as the δ18OH2O-SO4, and the K-Sr and K-F geothermometers, indicate subsurface temperature estimations for these thermal waters, which vary from 60 to 100°C.