Hydrogeological genetic pattern of the carbonate geothermal reservoir within the anticline folds in view of hydrogeochemical evolution and regional hydraulic connections

The hydrogeological genetic pattern of the carbonate geothermal reservoir within anticline folds represents a significant research topic in both geothermics and karstology. This study integrated environmental tracers with the anticlinal structures to elucidate the hydrochemical evolution and potential regional flow pathways of geothermal waters. From the Wentangxia anticline (WTXA) (northwest) to Taozidang anticline (TZDA) (southeast), the karstification degree decreased, while the estimated geothermal circulation depth increased, from which it can be inferred that geothermal water flows from northwest to southeast. Along the geothermal water flow direction, the average circulation depths ranged from 2580 m to 3627 m, while the average temperatures varied between 71 °C and 92 °C. In sequence, hydrochemical types identified along the recharge zone, runoff zone, and discharge zone were HCO 3 ·SO 4 Ca, SO 4 Ca, and either SO 4 Ca or SO 4 Ca·Mg, SO 4 2– and Sr initially increased and then decreased, while HCO 3 – exhibited an opposite trend. The stable 87 Sr/ 86 Sr ratio, along with elevated δ 13 C DIC and δ 34 S SO4 values in geothermal water, and the observed correlations among HCO 3 – , Sr, Ca 2+ , and SO 4 2– , indicated that the hydrochemical evolution is primarily driven by the dissolution of carbonates and gypsum, whereas sulfate reduction occurred secondarily in an anoxic environment influenced by overlying coal-bearing strata. The hydrogeological genetic pattern of geothermal reservoir was constructed by integrating geological background conditions, spatial karstification degree, regional geothermal water flow field, and hydrogeochemical evolution. These findings validate the use of hydrochemical tracers in indicating hydrogeological functions of carbonate geothermal reservoir and provide insights for promoting the sustainable development of geothermal resources.