The application of geochemistry to bicarbonate thermal springs with high reservoir temperature: A case study of the Batang geothermal field, western Sichuan Province, China

Abstract

The Batang geothermal field, located at the north-eastern margin of the Qinghai-Tibet Plateau, is a high-temperature geothermal system with two main geothermal areas, namely Bogexi and Rekeng. The Rehai geothermal field is also a high-temperature geothermal system, with a magmatic heat source which shows geothermal manifestations similar to those in Batang, especially in the Rekeng area. In this study, we compared the chemical compositions of different thermal springs in the Batang geothermal field with those in the Rehai geothermal field, to ascertain whether the thermal spring waters in Batang are also influenced by magma degassing or not. Further, we investigated the hydrogeochemical processes involved in forming the various thermal springs in the Batang geothermal field. We find that the thermal waters in the Batang geothermal field are not influenced by magma degassing. Although inorganic carbon, including HCO3− and CO32−, is the dominant anion component of most of the thermal waters in the Batang geothermal field, SO42− is present in variable amounts. In this area, the mixing of the thermal waters with steam containing some amount of H2S generated by metamorphism probably causes the relatively high SO42− concentration in the neutral thermal springs. Also, beneath the Rekeng area, there is a deep reservoir with the temperature of about 180 °C; a shallow reservoir of a relatively low temperature of about 75 °C was also identified in the eastern part. About half of the thermal spring waters in Rekeng directly originate in the deep reservoir, mixing with little or no snow-melt water and steam. Conversely, no steady reservoir exists beneath the Bogexi area. We attribute the high levels of Ca2+, Mg2+ and TIC (total inorganic carbon) in the thermal spring waters in the Bogexi area to the carbonate geologic stratum distributed at the outlets of the springs.