Exploration of Supercritical Condition in the Los Humeros Geothermal Field

Abstract

Sub- and supercritical geothermal resources offer significantly higher energy densities compared to hydrothermal systems. Owing to their low viscosity and high enthalpy, supercritical fluids can transport substantially more heat, potentially increasing the energy output per well by up to an order of magnitude. However, accurately identifying and distinguishing these high-temperature resources from traditional hydrothermal systems remains a major exploration and characterization challenge. Joint interpretation of electrical resistivity and gravity data has become standard in hydrothermal exploration, as each method is sensitive to different physical properties—resistivity to fluids and alteration, gravity to density contrasts and structure. Joint inversion can reduce the inherent non-uniqueness of individual methods by enforcing structural or petrophysical links between models. At the Sorik Marapi field (Sumatra), a 3D joint inversion of MT and gravity data, incorporating fault discontinuities, improved delineation of the graben structure. Gravity resolved lateral geometry, while MT provided depth resolution (Soyer et al., 2020). In Los Humeros (Mexico), combining gravity and surface-wave dispersion data led to better-constrained velocity and density models than from separate inversions (Carillo et al., 2024). Nevertheless, some studies opt for so-called cooperative joint inversion approaches, such as cross-gradient or structure-coupled constraints, which offer a flexible alternative (Um et al., 2023). Recent laboratory experiments at temperatures ranging from 25 to about 350°C, electrical conductivity increases because of both increasing surface and electrolytic conduction (Nono et al., 2020). Under supercritical conditions, i.e. temperature from 374°C to 600°C, electrical conductivity strongly decreases due to the evolution of water density and dielectric constant that affect both surface and electrolyte conduction. Apart from amphibolite, crustal rock conductivities at temperatures between 500°C and 700°C lie within the range of dry rock electrical conductivity values. With the aim of identifying geophysical signatures associated with sub- to supercritical temperatures, this study, compares gravity and electric resistivity of the Los Humeros geothermal field. The Los Humeros geothermal field hosts several wells with temperatures that suggest the potential presence of supercritical fluids. While gravity data help delineate key structural features, electrical resistivity is used to characterize thermally relevant zones. Our interpretations are ed against temperature measurements reported by Espinosa-Paredes and García-Gutiérrez (2003).