Hydrochemical evolution of karst geothermal water in the Heze uplift geothermal field, Shandong Province

The Heze uplift geothermal field is located in the tectonic unit of the buried uplift, Heze-Yanzhou buried fault uplift, Heze buried uplift, with a total area 6，330 km2. It hosts Ordovician crack karst thermal reservoirs of layering zone-like shape, with water yield of 80-402 m3.h-1 in single wells, depths of water level 20-55 m and wellhead temperatures 45-75 ℃.The thermal reservoir caprocks are Quaternary, Neogene and Permian-Carboniferous with a thickness of 600-2，500 m.Deep faults such as the Liaokao,Juye,Tianqiao and Fushan faults connect heat sources at depth, forming heat source channels. Secondary faults of these major deep faults developed well, nearby which limestone has undergone many tectonic movements and dissolution, resulting in the development of fissure karst which is the circulation, migration channel and water storage space of deep karst hot water as well as the main water source channel.The purpose of this work was to study the evolution characteristics of hydrochemistry of geothermal water.Two sections A-A′ and B-B′ were selected in the geothermal field.Piper,Schoeller and Na-K-Mg equilibrium diagrams and saturation index calculation were used to characterize the distribution and evolution of each chemical index in geothermal water. Results show that along the profiles A-A′ and B-B′, karst water turns from cold into hot, the hydrochemical type evolves from SO4·HCO3-Ca·Mg·Na or SO4·HCO3-Ca·Mg to SO4-Ca·Na and SO4-Ca. Mineralization increases gradually, HCO3-concentration decreases gradually, and SO42-, Ca2+ and K+ concentrations grow gradually. It reflects the evolution and supply characteristics of karst cold water to karst hot water. The karst cold water and hot water samples are located in different areas on the hydrogeochemical trilinear map, which shows that the cold water is supplied to the hot water. With distance supply area from near to far from the supply area, the degree of water rock interaction gradually intensifies, the saturation index gradually increases, and dissolution of karst hot water gradually becomes larger.