Geochemical evidence for the nonexistence of supercritical geothermal fluids at the Yangbajing geothermal field, southern Tibet

Exploring and exploiting high-temperature (even supercritical) geothermal resources are significant to meet energy demands and reduce carbon emissions. The Yangbajing geothermal field is the most exploited in China, with the currently highest temperature (329.8 degrees C) measured in a geothermal well. However, whether there are supercritical geothermal fluids beneath the deep parts of this geothermal field is under controversy. In this paper, the water isotope, chemical compositions, and C-He isotopes of gas samples were collected and analyzed. The geothermal water originated from the mixing of meteoric water and magmatic water (25%). The sources of CO2 in the geothermal field were dominated by the thermogenic degassing of carbonates and metasediments in the crust while the radioactive decay of U and Th in granite is the dominated source of He. The temperatures of three different reservoirs are 150 +/- 15 degrees C, 250 +/- 10 degrees C, and -320 degrees C (with a depth of -8 km), respectively. These were obtained using dissolved gas, soil CO2 flux, and noble gas geothermometers. Unlike other supercritical geothermal fields worldwide with larger, shallower, basaltic magma chambers, the Yangbajing geothermal field has a deep-seated, small-scale, granitic magma chamber. Thus, its geological conditions are not conducive for gestating supercritical fluids. These results are of great significance for exploring and developing hightemperature (even ultra-high-temperature) geothermal resources in China.

Automated summary

This study investigates the origin of geothermal water in the Yangbajing geothermal field, as well as the sources of CO2 and He in the field. The research identifies three different reservoirs with varying temperatures and determines that the geological conditions of the Yangbajing geothermal field are not conducive for supercritical fluids, unlike other geothermal fields worldwide. These findings are significant for exploring and developing high-temperature geothermal resources in China.