Comparative analysis of the genesis models of different geothermal reservoirs in Chengning uplift area in northwest Shandong based on hydrochemical isotope technology

CUI Rui; WANG Xuepeng; FENG Bo; LIU Xiyao; FENG Shoutao; LIU Shuai

doi:10.11932/karst20230513

Abstract：The Chengning Uplift Area is one of the important areas rich in extremely abundant geothermal resource in China.At present,geothermal resources in the study area are mostly used in the fields such as bathing,medical treatment,and heating.According to the existing geothermal geological data and analysis results,the total geothermal resources in this area are 1.70×1020 J,equivalent to 57.86×108 t of standard coal.Therefore,elucidating the genesis model of geothermal fields in this area is of great significance for the sustainable development and utilization of geothermal resources.This study uses hydrochemical isotope technology to compare and analyze the genesis of the sandstone thermal reservoir of Guantao Formation and the Cambrian-Ordovician karst thermal reservoir in the Chengning Uplift Area. The research results indicate that the cations in the geothermal water from sandstone and karst thermal reservoirs in this area are mainly Na+,and the anions are mainly Cl-.The mineralization degree of geothermal water in sandstone thermal reservoir is 4.19-5.96 g·L-1,with a pH value of 7.35-9.43,indicating neutral to weakly alkaline water.The mineralization degree of geothermal water in karst thermal reservoir is 5.91-11.10 g·L-1,and the pH value is 6.50-7.29;therefore,it is classified as neutral water according to its acidity and alkalinity.The geothermal water from sandstone thermal reservoir in this area is a product of atmospheric precipitation infiltration and replenishment during geological history,supplying water by lateral runoff.However,the geothermal water in karst thermal reservoir does not directly come from the nearest infiltration recharge of atmospheric precipitation,but through a longer distance runoff process,which has an obvious phenomenon of hydrogen and oxygen drift.The two sets of elevation of geothermal water supply,temperature of thermal reservior and depth of hot water circulation are respectively 459 m and 557 m,66 ℃ and 72 ℃,and 1,420 m and 1,795 m.From this,it can be seen that geothermal water in the study area is in a well-sealed geological environment,without shallow water mixing,and is deep circulating geothermal water. In addition,the research results also reveal that the supply area of geothermal water of Chengning Uplift is located in the Mount Tai area,and the heat source of its geothermal system is the heat flow conducted from the deep crust and a small part from the upper mantle.The deep fault in the study area generated a certain amount of frictional heat during its active period,and also served as a good channel for underground heat flow,connecting and conducting upwards the heat generated by magma in the deep crust and upper mantle.In addition,the area is a sedimentary basin with deep depression,which generates gravity compression heat under the pressure of the thick Meso-Cenozoic sedimentary layer.The heat generated by these sources is stored in the pores and cracks of the thermal reservoir under the thermal insulation effect of the cover layer with strong thermal resistance and poor thermal conductivity,and is the main heat source for the formation of hot water in the area.