Study on the hydrochemical characteristics and heat-accumulation mechanisms of karst geothermal reservoirs in Pingdingshan Coalfield

【Objective】Sedimentary basin-type geothermal systems host the most widely distributed and potentially vast geothermal energy resources in China. For a long time, however, there has been a lack of in-depth domestic research on the hydrochemical processes of geothermal fluids in low-to-medium temperature non-silicate reservoirs, as well as the applicability of traditional geothermometers.【Methods】Taking the Pingdingshan Coalfield as an example, based on hydrochemical data measured from 16 deep boreholes, this study investigated the hydrochemical processes of the deep karst geothermal water using hydrogeochemical approaches in combination with the in-situ hydrogeological conditions and geothermal geological background.【Results】The results demonstrate that the water-rock interaction equilibrium degree of the deep karst geothermal water in the study area is remarkably low relative to silicate minerals, indicating that traditional hydrochemical geothermometers cannot be utilized to estimate its deeper reservoir temperatures. Furthermore, the dissolution of dolomite and calcite constitutes the main source of Ca and Mg in the deep karst geothermal water in Pingdingshan. Simultaneously, hydrochemical evidence indicates that the deep geothermal water near the No.2 and No.11 coal mines may primarily originate from the direct recharge of surface water or meteoric water.【Conclusion】The geothermal anomaly zone located in the Cambrian limestone bulge area of the Pingdingshan Coalfield mainly relies on deep terrestrial heat flow for conductive heating, without any direct influence from additional heat sources such as magma. Local convection occurs within the karst geothermal reservoir, thereby forming a composite geothermal system superimposed by sedimentary strata and latent bulges. This study clarifies the hydrochemical processes of the karst geothermal water, as well as the heat sources and heat-accumulation and control mechanisms of the geothermal system in this area, which provides a crucial scientific basis for the subsequent rational development and utilization of local geothermal resources. In addition, it also enriches the applicability theories of traditional geothermometers for low-to-medium temperature non-silicate reservoirs.