Anatomy of the convective geothermal system from geophysical and hydrochemical data: A case study from the Changshou geothermal field, South China

There are abundant low-medium temperature geothermal resources in southern China; however, a lack of research into their properties restricts the utilization of the geothermal energy. To expand the geothermal energy potential in southern China, it is vital to clarify the nonvolcanic geothermal system mechanisms. This study aimed to elucidate the mechanisms for the Changshou geothermal field, as a representative structurally controlled convective geothermal system, by analyzing the geophysical and hydrochemical data. The surface-to-subsurface (400 m depth) electrical resistivity structure of the geothermal field was obtained by 2D inversion of two intersecting controlled-source audio-frequency magnetotellurics and electrical resistivity tomography geophysical profiles. We identified the cap rock, reservoir, and fluid pathways of the geothermal system and verified them via drilling results. The cap was composed of Quaternary sediments and Cretaceous sandstone with moderate resistivity (10-50 Om), and the geothermal reservoir and fluid pathways were composed of dolomites and fracture zones with low resistivity (less than 5 Om), respectively. The hydrochemical results showed that the geothermal water was of the type HCO3 center dot SO4-Ca center dot Mg, with low values of total dissolved solids resulting from shallow groundwater mixing. Isotope analyses indicated that the geothermal water was derived from precipitation in the Dahong Mountains, southeast of the geothermal field. The average temperature of the geothermal reservoir, as estimated by chalcedony and K-Mg geothermometers, was 53 degrees C. Based on the geothermal gradient calculation, the circulation depth of the groundwater was 1500 m, with the circulation process driven by heat flows originating deep within the earth. The Changshou geothermal field was identified as a potential source of energy for residential heating in winter, owing to the large flow (2700 m3/d) and high permeability (24.03 m/d) of the geothermal reservoir.

Automated summary

Southern China has abundant low-medium temperature geothermal resources, but the lack of research on their properties hinders the utilization of geothermal energy. This study aims to investigate the mechanisms of the Changshou geothermal field, a representative structurally controlled convective geothermal system, by analyzing geophysical and hydrochemical data. The study identifies the geological structure, thermal reservoir, and fluid pathways of the geothermal system, providing insights into its potential as a source of energy for residential heating in winter.