Research On Genesis And Development Of The Geothermal System In The Kangding-moxi Segment Of The Xianshuihe Fault

As sustainable green and clean energy,geothermal resource plays a significant role in the ecological environmental protection and social society development.The Kangding-Moxi segment of the Xianshuihe fault is located at the eastern Tibetan Plateau and belongs to Western Sichuan geothermal belt,reserving abundant geothermal resource.After the collision between Indian plate and Eurasia plate,complicated geothermal and geological setting was developed in the area,increasing the difficulty of exploition and utilization.The point is the mechanism how the distribution of geothermal resource is controlled by the Xianshuihe strike-slip fault is unclear,and the elements of geothermal system?e.g.,cover,reservoir,channel and source?are short of comprehensive knowledge.Therefore,the genetic conceptual model of geothermal system has yet to be improved,which can provide crucial backup for the exploitation and utilization for geothermal resource in the Kangding-Moxi segment of the Xianshuihe fault.This paper was supported by the practical subject of the geothermal resource in the Kangding Moxi area and scientific project of state key lab.Regional geological survey and sample collecting were conducted firstly.Spatial character of structural fractures were analyzed by bootstrap and semivariable function,discussing the controlling of the strike-slip fault system on the distribution of geothermal resource.Then,the water-rock interaction,recharging source,mixing process,and occurrence environment were investigated using hydrogeochemical information of the geothermal system.Furthermore,cover,thermal reservoir,channel and source were delineated to construct genetic conceptual model.Resource quantity was calculated by natural flow and volume methods,water quality,corrosivity and CaCO₃ scaling were evaluated.Based on the systemic analyses,major achievements were accomplished as below:?1?Under the compresso-shear stress mechanism,the distribution of geothermal water in the Kangding-Moxi area is significantly controlled by the Xianshuihe strike-slip fault system.Geothermal springs were mainly exposed in the Laoyulin district of the pull-apart basin,Hailuogou district of fault intersection and flank of anticline,and Caoke district of negative flower structure.The better development and continuity of fractures grow,the larger number of thermal springs are exposed?2?Hydrochemical types of thermal springs in the Laoyulin district present obvious zonation.Thermal springs in the middle part is typical of Na-HCO₃-Cl type,as well as borehole thermal water,while thermal springs in the margin belong to Na-HCO₃ type.Borehole thermal water and thermal springs in the Hailuogou and Caoke districts are characterized by Na-HCO₃ type.?3?The correlation analysis for major elements,relationship between Cl^-and trace elements,and rare earth element distribution patterns suggested Laoyulin borehole thermal water was produced in the granitic reservoir with inputs of mantle-derived components and recharged from the area with 4248-4292 meters high,while thermal springs and Hailuogou borehole thermal water were originated from carbonate metamorphic reservoir and recharged from the area with 2835-3866 meters high,mixed with 69%-72%cold water.Based on the SiO₂,cation,and geochemical-thermal-dynamic geothermometers,the deep granitic and carbonate metamorphic reservoir temperatures have a range of 232-235 ^oC and 201-205 ^oC,respectively,while shallowmetamorphic reservoir temperatures range from 143 ^oC to193 ^oC.?4?The genetic conceptual model for geothermal system can be concluded as follows:Excellent geothermal and geological background of the western Sichuan geothermal belt provides the large potential for the formation of geothermal water.Snow-melting water from the Mt.Gongga was infiltrated and then heated by the radioactive elements?U,Th and K?from the Yanshanian Gongga granitic batholith,partial melting flow in middle crust,and frictional heat of the strike-slip fault.Deep geothermal water with Na-HCO₃-Cl type was formed in the granitic reservoir at the depth of 4.7 km in the Laoyulin district,while deep geothermal water with Na-HCO₃type was formed in the carbonate metamorphic reservoir at the depth of 3.8 km in the Hailuogou and Caoke districts.Then,shallow geothermal water with Na-HCO₃ type was formed in the carbonate metamorphic reservoir at the depth of 50-300 m,mixed with cold water of Ca-HCO₃ type.Finally,shallow geothermal water was exposed as thermal springs in the Laoyulin district of the pull-apart basin,Hailuogou district of fault intersection and flank of anticline,and Caoke district of negative flower structure.Fractured granite and schist filled with calcite cementation and dense Quaternary deposits act as the cover of geothermal system.?5?The annual exploiting potential of thermal springs is 1.225×1014 J,calculated by natural-flow method.The annual exploiting potential of the geothermal water in the Laoyulin district is 5.3×1016 J,calculated by volume method,which can satisfy the 30-year operation of installed capacity of 56 MW.Geothermal water cannot be drunk directly,be used as medical mineral water,and not be scalled easily.Geothermal water has great prospecting potential in the area close to the Moxi village where geothermal water of 41 ^oC temperature and 400 m³/d flow has been achieved.