The Gonghe Basin locating in the Northeast Tibetan Plateau contains layers of low-to-median-temperature geothermal reservoirs, underlain by the granite hot dry rock (HDR) with temperature exceeding 180°C at the depth of 3000 m. It has been considered as a potential EGS demonstration site for the HDR exploitation in China. To understand the geothermal processes and to estimate the quantity of exploitable thermal resources in the Gonghe Basin, water and granite samples were collected and measured for chemical and isotopic compositions. δ18O and δD in water samples obtained from the upper geological layers suggested that the thermal water in the Gonghe Basin is recharged from meteoric water in the South-Qinghai Mountains. The maximum residual time of the thermal water in deep reservoir reaches 40000 years by 14C dating. The 3He/4He value in thermal water collected in deep reservoir is 4.65-9.52 ×10-8, suggesting radiogenic heat source in the crust of earth. The 87Sr/86Sr was measured to be 0.710987-0.712655, which indicated that strong interaction between geothermal water and granite at temperature more than 100°C. Temperature distribution in deep geothermal reservoir was determined by using geothermometers based on chemical ions in water, with the maximum reservoir temperature at 2440m estimated at 151°C.

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