EVALUATION OF GEOTHERMAL SYSTEMS IN TWO TECTONIC DEPRESSIONS IN CENTRAL MEXICO

Mexico is a country that has geothermal resources of high, medium and low temperature. The objective of this research is the evaluation of two geothermal systems: 1) An area located near The Chapala Graben (CG) in the western part of the Trans-Mexican Volcanic Belt (TMVB), on a tectonically active zone formed by regional system of grabens and half grabens, and 2) The Villa de Reyes Graben (VRG) that is part of a series of tectonic structures present in the Mesa Central physiographic province.

For the characterization of geothermal systems, sampling campaigns carried out (in dry and rainy season), where groundwater and rock samples were collected. The surficial temperature water range in the CG were from 18° to 75°C. The water´s chemical composition found allowed to identify two groups: a) Na⁺-K⁺-SO₄^2- to HCO₃ (temperature above 36 °C) and b) Na⁺-K⁺-HCO₃^- (18 °C – 36°C). Thermal manifestations of group B are in partial equilibrium with the rock, according to the Giggenbach diagram (relative concentrations of Na, K, and Mg). The chemical geothermometers shown that the probable reservoir temperatures range from 140° to 160° C for the thermal springs, and from 80° to 120° C for thermal well.

For the VRG, the temperature of the water in the discharge of the manifestations varies from 20° to 43° C. Through the water´s chemical composition were identified two groups: a) Ca²⁺-Mg²⁺- HCO₃^- and b) Na⁺-K⁺-HCO₃^-. Since most of the manifestations of this thermal system are not in equilibrium with the rock, the chemical geothermometers should not be used to determine the temperature in depth. However, the chalcedony geothermometer was applied in order to calculate the groundwater reservoir temperature of the last equilibrium; the values obtained for this area were from 80° to 90° C. The chemistry of the water shown anomalous concentrations of uranium and fluoride.

The preliminary conclusions of this research allowed identify that the thermal system in CG is probably a medium temperature geothermal system, of extensional domain type due to its geological characteristics in a tectonically active zone with young volcanism. The thermal system in VRG is a low temperature geothermal system with ion exchange chemical processes. However, a possible heat source could be radioactivity due to the uranium content.