Hydrogeochemical characteristics of Ordovician karst water in North China coalfield and its indicative significance for geothermal energy

Ordovician karst water in the North China coalfield is a green energy source that integrates water and heat. Therefore, it is of great significance for the exploration and development of geothermal resources to clarify the formation and evolution characteristics of the recharge source, geothermal reservoir environment, flow and circulation affected by coal mining. The Huaibei coalfield, one of the typical North China type coalfield, was selected as the study area on the basis of full understanding of its geological, hydrogeological and geothermal geological conditions. The hydrochemistry and isotope data and the previous study results were jointly employed to reveal the hydrochemical characteristics of geothermal water and the mechanism of water-rock interactions. Also, they were used to identify the recharge source of geothermal water, evaluate the temperature of geothermal reservoir and the depth of geothermal water circulation, and propose the formation and evolution model of geothermal water in the study area. The results show that the geothermal water of the Zhahe mining district is mainly recharged by precipitation infiltration, and the hydrochemical type and water-rock interaction are dominated by HCO₃−Ca and leaching dissolution respectively, indicating that the geothermal water in this area has a rapid renewal rate and a short circulation path. In the Suxian and Linhuan mining districts, the geothermal water is only recharged by precipitation with evaporation infiltration in some areas and is obviously mixed with ancient groundwater. There are various hydrochemical types including SO₄−Ca, SO₄−Ca·Mg, HCO₃·SO₄−Ca·Mg, SO₄−Ca·Na, Cl·SO₄−Na, Cl−Na, etc. Meanwhile, the water-rock interactions are dominated by leaching and dissolution and cation alternating adsorption (positive and negative), accompanied by dedolomitization, indicating that the geothermal water in this area has a long residence time, a long circulation path, and a more closed reservoir environment. The thermal reservoir temperature range from 34.8 to 69.1℃ according to the calculation of quartz and improved SiO₂ geothermometer, and the circulation depth range from 881 to 2281 m. The formation and evolution of karst geothermal water in the Huaibei coalfield is obviously controlled by structure, and there are significant spatial differences in geothermal water circulation and renewal capacity, reservoir temperature, circulation depth and hydrochemical environment. Therefore, the development and utilization of karst geothermal resources should comprehensively consider the regional characteristics.