Heat Source Mechanism Of Jibei Geothermal Field Revealed By Gas Geochemistry

Geothermal water is a special type of groundwater with high natural temperature and special chemical composition or gas.It is not only a thermal resource,hot mineral spring resource,but also a precious water resource.Based on the composition,abundance and isotopic composition of gases in geothermal system,the source of geothermal fluids can be identified and the formation mechanism of heat source can be traced.The characteristics of dissolved gas components of geothermal water and stable isotopes of hydrogen and oxygen indicate that the geothermal water in the north Ji’nan geothermal field mainly originates from the infiltration of atmospheric precipitation in the south,and is formed through deep circulation heating in a relatively closed geological environment.The CO₂in the geothermal fluid is mainly derived from the organic carbon in the surrounding sedimentary rocks,and the CH₄formation is mainly thermogenic and biogenic.There is no evidence of large faults in the north Ji’nan geothermal field cutting into the upper mantle.The decay of radioactive elements in the crust is the main source of He in the geothermal fluids in the study area,and the proportion of He from the mantle source is relatively small and stable,no more than 5%of the upper mantle line,indicating that He in the geothermal gases is dominated by the crustal source,and the main heat generation is still the decay heat of U and Th elements in the crustal granite,with a small amount of He from the mantle origin.The north Ji’nan geothermal field is a geothermal system formed under the background of high heat flow in the Meso-Cenozoic faulted basins of North China,which is dominated by conduction,local heat convection and Ordovician limestone reservoirs.The mantle heat accounts for 36.41%～46.33%,and the upper mantle contributes to the heat formation of the north Ji’nan geothermal field,but the earth conduction heat is still the dominant heat.After the intrusion of"Ji’nan pluton"in Yanshanian,the tectonic faults were frequent but shallow,and the upper mantle was not cut,and there was no upwelling channel for a large number of mantle hot materials in the north of the pluton.The main source of mantle heat:The surrounding rock of"Ji’nan Rock mass"communicates with the contact zone and fracture zone of solvable rock in the deep crust and even the connection between the upper mantle and the shallow geothermal fluid.After obtaining the deep heat flow from deep circulating groundwater,it migrates along the high permeability rock mass,the contact zone of solvable rock and the fracture zone to the shallow part,forming the high heat flow zone.The groundwater migration carries the heat runoff to the north.The heat source mechanism of the north Ji’nan geothermal field is mainly earth heat flow surface conduction heat accumulation,supplemented by zonal convection heat accumulation of rock mass,fault and surrounding rock contact zone,and groundwater migration conduction-convection heat accumulation.