Subsurface stratigraphy and its correlation with the surficial geology at Los Humeros geothermal field, eastern Trans-Mexican Volcanic Belt

Los Humeros is one of the main Pleistocene silicic volcanic complexes and the third geothermal field in exploitation in Mexico. It is located in the eastern sector of the Trans-Mexican Volcanic Belt (TMVB) and has a long and complex geologic evolution involving the formation of at least two large nested calderas associated with very large ignimbrite-forming eruptions during the last 0.46 Ma (Ferriz and Mahood, 1984). Due to its young activity, it has an active geothermal field (Yáñez and García, 1980) producing about 65 MW. Several wells for both exploration and exploitation of geothermal fluids have been drilled in the caldera interior, showing the subsurface stratigraphy and structure of the volcanic center. A revised lithostratigraphic characterization of Los Humeros subsurface together with petrographic, geochemical and 40Ar/39Ar-based geochronology data support correlation of rock formations at depth with surface geology. This study shows the very heterogeneous nature of the subsurface stratigraphy of Los Humeros volcanic complex, helping to refine its internal structure and evolution. This provides a more realistic configuration of the heterogeneities of the geothermal reservoir and the vertical and lateral distribution of permeable conditions controlling the configuration of the geothermal reservoir. These results can be important to guide the prospection of potential areas for the possible expansion of the geothermal field.