Characteristics of the underground hot water circulation system western Yunnan Yingjiang basin and resources development potential

ZHANG Hua; HE Raosheng; KANG Xiaobo; YANG Yingbin; LI Qin; GAO Yu; ZHOU Junrong; CHAI Jinlong

doi:10.11932/karst20240609

Abstract：The geothermal energy of the Yingjiang basin is located within Yingjiang-Longchuan high-temperature hot water belt,which is part of Gaoligongshan-Tengchong sub-area of high-temperature hot water region of western Yunnan.The study area belongs to the southern extension of Gaoligong mountain,where the terrain gradually descends from the northeast to the southwest,resulting in an overall spatial pattern characterized by valleys and ridges.Basins,mountains,and rivers in the area are controlled by structure,showing a NE-SW trend.In addition,the Yingjiang basin is a fault basin formed by geological structure.Its geomorphology are divided into five categories:fault accumulations,low hills of lake terraces,shallow cutting gentle slopes of low or middle-height mountains,medium cutting steep slopes of middle-height mountains,and karst middle-height mountains.The Yingjiang basin is located in the eastern margin of the collision zone between the Indian Plate and the Eurasian Plate.Due to the active crustal movement for a long time,the geological structure in this area is complicated,the rock fold is broken and the stratum is missing.The outcrop strata are mainly composed of Lower Proterozoic Gaoligongshan Group(Pt1GL.),Devonian Guanguan Formation(D1g),Permian Bangdu Formation(P1b),Neogene Mangbang Formation(N2m)and alluvial and pluvial strata of the Quaternary(Q).The Yingjiang basin is rich in geothermal resources with a total of 38 geothermal points,and the geothermal outcrops can primarily be classified into 3 types:hot springs,hot wells,and open wells.However,the investigations and studies on geothermal resources are not sufficient,and especially the source of geothermal resources recharge,and the amount and potential of geothermal resources need to be illuminated.In this study,the geothermal geological structures,geothermal structures,boundaries,sources and pathways of geothermal recharge of the study area have been analyzed and studied through field investigations,basic geological analyses,mathematical models,and equations of atmospheric precipitation lines in China.Geothermal resources have been calculated by the method of heat release for hot springs and the summary of spring flow. According to the research findings,the cover layer in the study area is composed of multiple cycles from the coarse to the fine,arranged from bottom to top.The sedimentary environment is dominated by fluvial facies and lakefront facies,with a total thickness of 100-2,000 m,and some of them are more than 2,400 m,providing an environment for geothermal heat storage and preservation.The heat storage structure is belt-shaped,and its conditions are controlled by principle faults and concealed faults.The main layers for heat storage are composed of magmatic rocks,metamorphic rocks,granitic sand conglomerate,fine sandstone,magmatic rocks and metamorphic rocks of Neogene Manbang Formation,which provide the necessary conditions for the exposure of hot springs.At present,the highest temperature is shown in the borehole drilled by Dimete Company in the geothermal anomaly area of Lianhua mountain.The temperature in the borehole reaches 155℃.The layers of heat storage are composed of magmatic rocks,metamorphic rocks,granitic sand conglomerate and fine sandstone of Neogene Manangba Formation.The Yingjiang basin is characterized by strong neotectonic movements,structural development,and heat flow activities in the depth,which suggests that magma pocket is the source of heat energy in this area.Combined with the terrain conditions around the basin,it is considered that groundwater is recharged by the deep circulation of atmospheric precipitation infiltration in the mountainous areas around the basin,with the recharge distance of 2.0-9.9 km,and the recharge depth of 1.6-3.0 km.Groundwater permeates to the deep crust along the primary fissures and late tectonic fracture zones of weathered granite and metamorphic rocks,and then moves upward after being heated by deep heat sources to form geothermal fluids,which are stored in the pores,fractured aquifers and fracture zones at the bottom and edge of the basin,and are exposed to form hot springs and geothermal springs at the parts where geomorphologic features and structures are favorable.According to the principles of geology,structure,temperature and concentration of geothermal resources,four evaluation units are divided.They are the unit of geothermal resources at medium-high temperature(Ⅰ)in Lianhua mountain of the Yingjiang river in the northwest of the basin,the unit of warm water resources at low temperature(Ⅱ)in the Yingjiang river in the northeast of the basin,the unit of warm water resources at low temperature(Ⅲ)in Taiping-south slope in the middle of the basin,and the unit of warm water resources at low temperature(Ⅳ)in Nongmu-Manyun of Yingjiang in the south of the basin.Based on the existing parameters,it is estimated that the heat stored by geothermal fluids in the study area is 484.63×106 MJ,and the recoverable amount of geothermal fluids is 3.178 million m3·a-1.The study on geothermal genesis and the characteristics of circulation system of hot water in the Yingjiang basin has improved the research of deep geothermal resources in the Yingjiang basin and western Yunnan,providing resource guarantee for the exploration,development and utilization of geothermal resources,and providing the basis for the local government to confirm the registration and management of geothermal resources.