Geothermal fluid circulation in the Guide Basin of the northeastern Tibetan Plateau: Isotopic analysis and numerical modeling

The Zhacang geothermal field in the Guide Basin, in the northeast of the Tibetan Plateau, is considered a potential target for establishing a geothermal power plant. This study investigated the formation mechanisms of geothermal resources in this field by combining isotopic analysis (3H, D, 18O, 3He and 4He) and numerical modeling of fluid-heat-helium transport. δD and δ18O indicated that the meteoric water is the most likely source of the geothermal water in the Zhacang field, with depleted δD and δ18O in deeper water indicative of recharge via snow melt and/or from cooler climates. A west-east trending conductive fault acts as a conduit that transports this meteoric water from an overlying stream and the Quaternary aquifer to the fractured granites in the Zhacang field. The downward groundwater movement in this conductive fault is driven by gravity force, and water temperature increases with depth. An uneven temperature distribution induces buoyancy force which results in heated water being transported upward to the land surface at positions where the conductive fault intersects an impermeable north-south trending fault. The water mass balance between recharge and discharge rates and the numerical modeling of the helium ratio imply that there is no external fluid input from the deep subsurface to induce the abnormally high temperature in the Zhacang field. Instead, the heat is generated by the friction of fault activity and/or the remaining heat in granite formed in the Triassic. The numerical modeling of heat and flow transport yields a circumfluence flow pattern in the fault zone, where a stagnant flow zone with high groundwater residence time appears. Both flow velocity and temperature distribute unevenly in the fault and never reach a steady state under the joint influence of gravity force and buoyancy force.