Hydrochemical characteristics and genetic implications of fault-uplift type carbonate geothermal reservoirs in Hebei Province, China

This paper is the result of geothermal survey engineering.

Objective The Xianxian Uplift in the southern Cangxian Anticline hosts abundant low-medium temperature geothermal resources, representing a typical sedimentary basin geothermal system dominated by heat conduction. Investigating the hydrochemical characteristics of geothermal water in this area is crucial for its rational exploitation and scientific management.

Methods Through the collection and analysis of water samples from geothermal wells in the Xianxian County geothermal field, this study comprehensively applies methods such as correlation analysis, hydrochemical diagrams, and principal component analysis to investigate the origin and evolutionary processes of geothermal water in the carbonate rock thermal reservoir of the region.

Results The results indicate that the majority of geothermal water in the study area is slightly alkaline, with TDS(Total dissolved solids) levels ranging from 5,581 to 6,743 mg/L, classifying it as highly mineralized water. All samples are of the Cl-Na hydrochemical type. The Na-K-Mg equilibrium diagram and PHREEQC calculations reveal that water–rock interactions in the study area have not reached equilibrium. Carbonate minerals such as calcite and dolomite exhibit saturation indices greater than 0, while evaporite minerals like halite and gypsum have saturation indices less than 0. Minerals such as quartz, chalcedony, and fluorite show saturation indices close to 0, indicating a tendency toward equilibrium. The quartz geothermometer and the multi-mineral equilibrium approach yield estimates of reservoir temperature that are closer to the actual geothermal reservoir temperature.

Conclusions The geothermal water in the Xianxian County geothermal field exhibits a singular hydrochemical type and a high degree of concentration. The contents of trace elements such as Li⁺, Sr²⁺, and F⁻ far exceed the limit standards for mineral water. Water–rock interactions have not reached equilibrium. The ionic composition is governed primarily by the dissolution of minerals such as halite, carbonate rocks, and gypsum, and is also influenced by cation exchange. The reservoir temperature ranges between 80 and 100 °C. These findings provide critical data and theoretical support for geothermal resource evaluation and sustainable utilization in the region.