Deep Thermal Structure And Geothermal Origin Of Xianxian Geothermal Field

One of the most significant geothermal fields in this area is the Xianxian geothermal field,which is situated in the center of the Cangxian Uplift.It has plenty of geothermal resources and favorable geological conditions for geothermal energy.However,the source and formation of hot water are not clear,and the deep thermal structure and thermal evolution mechanism are not clear.Therefore,it is urgent to establish a complete conceptual model of geothermal genesis to provide support to direct the use of deep geothermal resources and their exploitation.This thesis has carried out a large number of deep well temperature measurement,fine rock and soil thermal property testing,collected geothermal fluids in the area for testing and analysis of hydrochemistry,stable isotopes and other aspects.Combined with the geological conditions of strata and structures in the area,The geothermal system in this area is systematically studied from the aspects of heat source,geothermal fluid recharge source,geothermal fluid migration channel and heat storage characteristics.The region’s deep thermal structure and geothermal genetic process have been improved by thorough investigation.The main understanding is as follows:The analysis of water chemistry data shows that the deep underground hot water in Xianxian geothermal field belongs to brackish water,the water chemistry type is Cl-Na water,and the water-rock interaction is not balanced.The main ion content and mineralization degree of geothermal water in the area are proportional to the burial depth.The degree of geothermal fluid mineralization increases with the depth of burial.The formation of geothermal water in the area is affected by the infiltration of atmospheric precipitation,which is continental eluvial water.Deep hot water has a long flow path,slow water circulation and deep metamorphism.Dolomite,calcite and gypsum are the main sources of Mg²⁺,Ca²⁺,HCO₃^-and SO₄^2-in the deep hot water of Xianxian geothermal field.Besides,other minerals containing calcium and magnesium are also part of the sources.The recharge source of deep hot water in the area is atmospheric precipitation,and the recharge elevation of geothermal water is about 619～1069m.The Jxw hot water in the area is between 16,000 and 18,000 years old.The sulfur in geothermal water mainly comes from the leaching of sedimentary layer.The thermal property of rock demonstrates that the thermal conductivity of rock in the Xianxian geothermal region is closely tied to formation lithology rather than having an obvious relationship with depth.The thermal conductivity of shallow cap is small,while the thermal conductivity of bedrock is large.The earth’s heat flow is 70.58 m W/m²,which is more than mainland China’s average heat flow.Crust heat flow q_c is16.36m W/m²,mantle heat flow q_m is 54.22m W/m²,q_c/q_m=0.3,which is a"hot mantle cold shell"type.Mantle heat flow accounts for the majority of the heat flow in the research area,and the heat produced by radioactive decay also contributes to some extent.According to the numerical results of the deep earth temperature,the depth of Curie surface in the study area is about 24 km,the temperature of Moho surface is about 749℃,and the thickness of hot lithosphere is about 85～96 km.The shallow portion of the crust can more easily transfer heat from the deep mantle due to the thin lithosphere.A full geothermal field system from bottom to top is formed by the Xianxian geothermal field,which has a reliable geothermal heat source,a thermal channel for conduction,a thermal reservoir,a thermal reservoir cap,and a geothermal fluid migration channel.The primary heat sources that move upward by heat conduction and heat convection of deep and wide faults are the radioactive decay heat generation of crustal thermal materials and mantle heat.The overlying sedimentary cover creates good medium-low temperature geothermal water supplies in the thermal reservoir with high porosity and high permeability aquifer and accumulates good thermal insulation for it.