An analysis of the endowment characteristics and geneses of geothermal resources in the Zhangye Basin

As a part of Hexi Corridor in Gansu Province, the Zhangye Basin is located in the middle reaches of the Heihe River Basin, with high topography in the southeast and low topography in the northwest. The existing exploration data show that the Zhangye basin is rich in hydrothermal geothermal resources. Based on the study of geophysical exploration, geothermal drilling, geo-temperature measurement and hydrogeochemistry, this paper analyzes the occurrence characteristics of geothermal resources and discusses the genetic mode in Zhangye Basin. The geothermal field in the Zhangye basin belongs to the sedimentary basin type of low-medium temperature. The geothermal reservoir is composed of sandstone and glutenite of Neogene Baiyanghe Formation with layered distribution. Temperature of the geothermal reservoir ranges from 47 to 82 °C calculated by using the potassium magnesium geochemical temperature standard. The caprock consists of mudstone of Neogene Shulehe Formation and Quaternary unconsolidated sediments. The geothermal water type is mainly clastic pore water. The characteristics of hydrogen and oxygen isotope indicate that the main recharge source is atmospheric precipitation in the southern Qilian Mountains. The deep faults in the northern margin of Qilian Mountains and NNW-trending basement faults in the basin are good conduits for deep circulation of the geothermal fluids. After receiving recharge, groundwater migrates along the water-conducting fault zones or rock pore fractures. Heating by the deep heat conduction, it occurs in the pores of clastic rocks and forms geothermal resources in this area. The results of hydrochemical analyses show that the geothermal water in this area belongs to continental sedimentary water containing dissolved water in rock salt formation, and the hydrochemical type is of Cl·SO₄—Na. The contents of fluorine, SiO₂, total dissolved solids and total hardness increase with the increasing water temperature. The tritium value of hot water in the area is generally less than 2.0 TU, indicating that the formation age is relatively early. The results of carbon-14 analysis further confirm that the formation age of the regional geothermal water is more than 20,000 years, reflecting the characteristics of a long geothermal fluid supply path and slow runoff. The research results can provide important reference for exploration and utilization of geothermal resources in the Zhangye Basin.