Study On The Characteristics And Formation Mechanism For Typical Geothermal Systems Of A Railway In Southeastern Tibet

The study area is located in the southeast of the Qinghai-Tibet Plateau in the collision and squeezing zone between the Indian Plate and the Eurasian Plate,which includes the hengduan mountain part of eastern Tibet and the alpine valley part of southeast Tibet with complex geological structure,and it is located in the Mediterranean-South Asian geothermal anomaly zone with strong hydrothermal activity.The high ground temperatures induced and controlled by the strong tectonic activity of the railway area restrict the construction of tunnels in the area.In this thesis,the geothermal energy of the Qamdo-Nyingchi section of a Railway is used as the research object to discuss the distribution of geothermal heat;according to the ionic components of hot and cold water in the research area,the hydrological geochemical characteristics of geothermal fluids are analyzed by using hydrochemical and isotopic geochemical analysis methods,the material sources and hydrogeochemical processes are summarized,and the recharge and heat storage of underground hot water are analyzed.Based on the above research results,combined with the geological background,remote sensing and drilling results of the research area,the types of heat storage and the movement and formation process of geothermal fluids are discussed,the formation mechanism of the typical hydrothermal systems in this section is summarized,and the conceptual model of the typical geothermal system in the study area is established.Through the study,the following conclusions have been drawn:（1）The geothermal anomaly zone along the Qamdo-Nyingchi section is mainly located in the Guxiang-Lulang section and the Qamdo-Luolong section.From the interpretation of geothermal anomaly results and hot spring distribution and characteristics analysis by thermal infrared remote sensing,the Guxiang-Lulang section through which the railway passes is most likely to encounter high ground temperature,while the Qamdo-Luolong section will encounter low and medium geothermal temperatures.（2）The distribution of hot springs,active faults and earthquakes is highly positively correlated,indicating that the surface geothermal and linear structural features have a good co-frequency effect with the deep thermodynamic features.Geothermal is mainly controlled by two sets of faults,the north-west deep fault and the associated secondary fault,which provide a good transport space and storage space for hydrothermal activity,and several deep faults provide a channel for the underground deep circulation of surface water,and the secondary fault provides a path for upward transport,forming a relatively stable convection system.（3）The temperature of the hot spring along the Guxiang-Lulang section in the study area is mostly between 40～90°C,which is medium and high temperature hot water.The p H is mostly weakly alkaline,and the salinity is between 200～3265 mg/L.The main anion components of water are HCO₃^-and SO₄^2-In the cation,the content of Na⁺in hot water is higher,followed by Ca²⁺and Mg²⁺,while the content of Ca²⁺and Na⁺in surface water is higher,and the content of Mg²⁺is less;The temperature of the hot springs in the Qamdo-Luolong section is mostly between 20～80°C,which is medium and low temperature hot water.The p H value is neutral or weakly alkaline,and the salinity is between 215～4226 mg/L.The hot water chemistry types can be divided into two categories,namely HCO₃-Na and SO₄-Ca-Mg,and the main water chemical type of surface cold water is HCO₃-Ca-Mg and HCO₃-SO₄-Ca-Mg.（4）The overall hydrochemical characteristics of hot water in the study area are that the higher the temperature,the higher the content of chloride ions,sodium and potassium ions,and the lower the content of calcium and magnesium ions.In the process of underground circulation,it is mainly affected by the dissolution and filtration of silicate and carbonate minerals.The Cl^-content of some hot spring water is much higher than that of surface cold water and other hot water,which is mainly affected by the dissolution and filtration of some salt-bearing strata in the Upper Triassic and the mixing of deep mantle-derived materials,and there are different degrees of hot and cold water mixing.（5）The equilibrium temperature of hot water heat storage in the study area is mostly located at 100～160°C,the equilibrium temperature of thermal storage in Guxiang-Lulang section is mainly between 120～160°C,and the equilibrium temperature of hot storage in Qamdo-Luolong section is mainly between 100～140°C.The heating depth of the hot water circulation is mostly located at 3100～4700m,which belongs to the deep thermal storage.The hot water circulation depth of the Guxiang-Lulang section is mostly located at 3500～4700m,and the Qamdo-Luolong section is mostly located at 3100～3800m.There is a good positive correlation between the depth and the thermal storage temperature.（6）A general overview of the formation mechanism of typical geothermal systems in the study area:Atmospheric precipitation and ice and snow melt water in nearby 4000～5000m mountain areas infiltrate along deep and large faults for a deep circulation of more than 3000m.In this process,it is mainly subjected to frictional heat generated by structural faults and high heat flow heating of deep heat sources.In the process of circulation,a strong physicochemical reaction occurs with the surrounding rock,and then part of it enters into structural fissures or bedrock fissures to form deep-buried band-shaped fissure-type heat storage,while a part migrates upward along secondary faults near deep and large faults to form shallow pore-type thermal storage or outcrops to form springs under the action of temperature and pressure.