Indication of hydrogen and oxygen stable isotopes on the characteristics and circulation patterns of medium-low temperature geothermal resources in the Guanzhong Basin,China

Guanzhong Basin is a typical medium-low temperature geothermal field mainly controlled by geo-pressure in the west of China. The characteristics of hydrogen and oxygen isotopes were used to analyze the flow and storage modes of geothermal resources in the basin. In this paper, the basin was divided into six geotectonic units, where a total of 121 samples were collected from geothermal wells and surface water bodies for the analysis of hydrogen-oxygen isotopes. Analytical results show that the isotopic signatures of hydrogen and oxygen throughout Guanzhong Basin reveal a trend of gradual increase from the basin edge areas to the basin center. In terms of recharge systems, the area in the south edge belongs to the geothermal system of Qinling Mountain piedmont, while to the north of Weihe fault is the geothermal system of North mountain piedmont, where the atmospheric temperature is about 0.2℃-1.8℃ in the recharge areas. The main factors that affect the geothermal water δ¹⁸O drifting include the depth of geothermal reservoir and temperature of geothermal reservoir, lithological characteristics, water-rock interaction, geothermal reservoir environment and residence time. The δ¹⁸O-δD relation shows that the main source is the meteoric water, together with some sedimentary water, but there are no deep magmatic water and mantle water which recharge the geothermal water in the basin. Through examining the distribution pattern of hydrogen-oxygen isotopic signatures, the groundwater circulation model of this basin can be divided into open circulation type, semi-open type, closed type and sedimentary type. This provides some important information for rational exploitation of the geothermal resources.