Hydrogeochemical characteristics and formation mechanism of the karst thermal reservoir at the northern edge of the Luzhong Uplift

The geothermal area in the northern edge of the Luzhong Uplift is located between Mount Tai and the northwest plain of Shandong. In terms of the geotectonic division, the study area is located in the Luzhong Uplift of the Luxi Uplift in the North China Plate, which is fan-shaped with the arc facing north in the plane, and it is a monoclinal structure with the stratum tilting gently to the north. There is a very thick layer of Cambrian-Ordovician carbonate strata in the northern edge of the Luzhong Uplift, which has the geothermal geological conditions for the formation of a large karst geothermal field. Under the influence of the collision of the Pacific Plate and Eurasian Plate, especially the tectonic activity on the rim of the Pacific Plate since the Mesozoic, NW and NE oriented fracture structures have been widely developed in the study area. This combination of tectonic structures with different ages and properties provides the preconditions for the formation of karst geothermal resources characterized by the convection-conduction between low and medium temperatures in this area. The main aquifer in the study area is the Cambrian and Ordovician carbonate aquifer, which is directly or indirectly replenished by atmospheric precipitation in the exposed karst area in the upper and middle reaches. Some of the precipitation is concentrated and flows out in a small area, and the remaining water continues to flow to form a karst water enrichment zone in the downstream piedmont, intermountain basins and hidden limestone distribution areas in the valleys.

The karst thermal reservoirs in the northern edge of the Luzhong Uplift in Shandong Province are rich in low-and medium-temperature geothermal resources, which are characterized by a large water yield and easy recharge. Thus, we can study the hydrogeochemical characteristics of the geothermal water to analyze the formation mechanism of the geothermal resources, which is of great significance in promoting the effective development and utilization of geothermal resources. In this study, 32 geothermal wells in the geothermal area in the northern edge of the Luzhong Uplift are studied, and the Piper diagram, Schoeller diagram, ion component ratio characteristics, isotopic characteristics, Na-K-Mg ternary diagram, mineral saturation index and SiO₂ geothermal temperature scale are used to analyze the recharge source, water-rock interactions, transport pathway, and cyclic evolution characteristics of the geothermal water.

The results show that in the evolution of karst groundwater into geothermal water, the hydrogeochemical types gradually change from HCO₃·SO₄-Ca and HCO₃-Ca to SO₄·Cl-Na·Ca, SO₄·Cl-Ca·Na, SO₄·HCO₃-Ca, HCO₃·SO₄-Ca and Cl·SO₄-Na·Ca, and the TDS content increases successively. The contents of ${\rm{SO}}_4^{2-}$, Ca²⁺, Na⁺, F⁻, Li, Sr and H₂SiO₃ increase continuously, while the percentage of ${\rm{HCO}}_3^{-}$ content decreases. The temperatures, chemical compositions, and ion contents and characteristics of the karst water formed in different sections of the geothermal field are different, which proves that the recharge sources, circulation depths, and circulation paths of the karst water at different depths and in different sections of the geothermal field are different. The karst water with a lower temperature comes from the shallow and middle circulation flow system, while the geothermal water with a higher temperature in the deep part comes from the deep circulation with a larger circulation depth and a longer path of the cycle.

The aquifers of the carbonate reservoirs in the study area are composed of limestone and dolomite deposited in a marine environment. The metamorphic coefficient (γNa/γCl), Cl/Br ratio, and desulfurization coefficient indicate that the aquifers of the carbonate reservoirs in the study area are poorly sealed, and the atmospheric precipitation gradually leaches the rock salt-bearing strata, so the geothermal water shows its characteristics of dissolved water. According to the Na-K-Mg equilibrium diagram, the hydrothermal water in the carbonate reservoir in the geothermal area at the northern edge of the Luzhong Uplift is in a non-equilibrium area and is not saturated. That is, the water-rock interaction has not reached the equilibrium state, and the dissolution is still in progress.

The temperature of karst thermal reservoir in the geothermal area is 39–70 ℃, and the circulation depth of geothermal water is 856–1,877 m. By calculating the thermal storage temperature and the circulation depth of the geothermal water and combining with the analysis of the characteristics of the temperature measurement curve, it was determined that there is not only a conduction-type geothermal system but also a convection-conduction-type system in the study area.

The geothermal water samples from the study area are distributed near the regional precipitation line, and the stable hydrogen and oxygen isotope compositions are similar to those of modern atmospheric precipitation, indicating direct or indirect recharge via the infiltration of atmospheric precipitation. According to the ¹⁴C ages of the geothermal water at the northern edge of the Ludong Uplift and the significantly negative hydrogen and oxygen isotope compositions of the geothermal water, the main source of the geothermal water supply is paleo-atmospheric precipitation in the southern mountainous area under the cold climate conditions during the Late Pleistocene, and after infiltration, the subsurface runoff is heated by the Earth's heat flow through deep circulation.