Genetic mechanism of geothermal water in the sandstone reservoir of western Sichuan, SW China: Evidence from hydrochemistry and δD − δ18O isotopes

Abstract

Geothermal energy is the vital clean and renewable energy in the world. In China, most of high−temperature sedimentary − type geothermal waters are developed in western Sichuan, such as geothermal springs in the Litang county. In this study, hydrochemistry and δD − δ18O isotopes were investigated to clarify the genetic mechanism of high−temperature sedimentary − type geothermal waters in the Litang county. The self-organized map and Piper diagram showed three hydrochemical types of geothermal waters: (1) HCO3 − SO4 − Na, (2) HCO3 − Na with low Cl− concentration, (3) HCO3 − Na with high Cl− concentration. Cluster 1 geothermal water of Na − HCO3 − SO4 was dominated by dissolution of silicate and sulfate minerals. Cluster 2 geothermal water of HCO3 − Na with low Cl− concentration was affected by dissolution of silicate and CO2 degassing and mixed by large amounts of shallow cold water. Cluster 3 geothermal water of HCO3 − Na with high Cl− concentration was affected by dissolution of sandstone silicate and CO2 degassing and mixed by less shallow cold water. The recharge elevation of geothermal water in Litang ranges from 4799 to 4972 m, with an average of 4915 m. This suggests that the recharge sources of geothermal water are snow melt water and magmatic water. Cluster 3 geothermal water displayed the largest contribution of magmatic water. The temperature of HCO3 − SO4 − Na geothermal water after mixing is 92−102 °C, with an average of 97 °C; the initial temperature of HCO3 − Na with low Cl− geothermal water is 60−72 °C, with an average of 65 °C; the initial temperature of HCO3 − Na with high Cl− geothermal water is 74−91 °C, with an average of 81 °C. The temperature of geothermal water after mixing is Class 1 > Class 3 > Class 2. From the perspective of reservoir temperature, the Class 3 of geothermal water had the highest values, followed by Class 1 and Class 2 of geothermal water. From the perspective of cold − water mixing ratio, Class 2 and Class 3 of geothermal water had a similar mixing ratio, slightly higher than that of Class 1 of geothermal water. Therefore, the class 3 geothermal water with HCO3 − Na with high Cl− concentration possess the great potential for exploitation.