Origin Of Geothermal System In Baoshan Basin

As a clean energy source,geothermal energy is one of the hotspots of future energy development.It is of great practical significance to research the cause of geothermal system.The Baoshan Basin is located between the Nu River and the Lancang River suture zone,with complex structures,well-developed faults,good geothermal geological conditions,and abundant geothermal resources.However,due to the low level of regional research,there is no systematic understanding of the genetic mode and occurrence regularity of geothermal resources,which restricts the development and scientific utilization of geothermal resources in the region.Therefore,it is of great importance to ascertain the genetic model of geothermal resources and the characteristics of the geothermal system in the study area.This paper takes the geothermal resources of Baoshan Basin as the research object.Through collecting data,on-site investigation,hydrogeochemistry and other methods,we studied the geothermal geological conditions,deep structure,geothermal field characteristics,and geochemical characteristics of geothermal fluid in the area,and then determined the formation conditions and distribution of geothermal energy.We compared the characteristics of the geothermal blocks,established the crust-mantle thermal structure of the study area,and explored the replenishment and migration process of geothermal water and the mixing effect of cold and hot water.We also demonstrated five characteristics of the geothermal system:heat source,water source,channel,thermal reservoir,and caprock.Lastly,three types of geothermal system genetic conceptual models of the area are proposed:deep buried closed type,shallow buried mixed type and circulation open type.The specific research results are as follows:(1)Baoshan Basin is a rift basin formed by crustal uplift and stress release.It is a not only structural geological but also hydrogeological unit in this area.The geothermal in the basin is a low-temperature geothermal resource.The temperature of geothermal springs and wells is between24 and 97?,and the distribution is mainly attributed by the north-south-trending fault structure.(2)The heat flow is high,whose value is 65?75 m W·m^-2,and the geothermal gradient is between 3?6.4?/100m.Using data from different geophysical exploration methods,we explored the crust-mantle structure of the Baoshan Basin,and constructed a crust-mantle thermal structure model.The mantle heat's contribution to the heat flow was calculated to account for 61.6%,indicating that the mantle heat is the main heat source in the study area.(3)The analysis of the hydrochemical characteristics of geothermal water shows that the geothermal water in the basin is mainly weakly alkaline water,which reflects the characteristics of carbonate aquifers.The difference in TDS content is large,reflecting the difference in the mixing degree of hot and cold water between different blocks and the migration path of geothermal water.Among them,the deep-buried closed geothermal system has the largest TDS,and the circulating open geothermal system has the smallest TDS.The heat storage temperature is estimated by the cation temperature scale,SiO2 temperature scale,silicon enthalpy equation,and fixed aluminum method.The silicon enthalpy equation also shows that the mixing ratio of cold water is 43%?89.9%,and the deep heat storage temperature without cold water is 95?138?.(4)Based on the isotopic characteristics of geothermal water,it is shown that the recharge of groundwater mainly comes from the infiltration after rainfall in the mountainous areas surrounding the basin.The recharge elevation is 2115-2566 m,and the formation time is earlier than 1953.There is an oxygen drift phenomenon in?18O,which reflects that geothermal water has experienced long runoff and long-term retention in the ground.(5)Using Earth Volumetric Studio(EVS),a three-dimensional geological model of Baoshan Basin was established,showing the spatial distribution of geothermal resources under the influence of faults in the basin.Based on the temperature field,hydrogeochemical data,thermal storage temperature calculations and the mixing effect of cold and hot water,three genetic models of the Baoshan Basin's geothermal system are proposed:deep buried closed type,shallow buried mixed type and circulation open type.