Research On Genesis And Development Of Geothermal System In Wanjiang Economic Belt

The Wanjiang Economic Belt is located in the central south part of Anhui Province.It is controlled by fault structure and its activity.Uplifted mountains and faulted basins develop alternately,which creates favorable geological conditions for the formation and occurrence of geothermal resources in the region.Thus,the district is rich in geothermal resources.This article took geothermal resources in Wanjiang Economic Belt as the research object.Through studying the distribution of geothermal resources,geothermal field and hydrogeochemical characteristics of geothermal water in this area,this study clarified the control factors and formation conditions of the geothermal system in the area,and revealed the sources of geothermal water supply,the environment and the water-rock interaction and mixing during its circulation.Also,this study clarified the characteristics of caprocks,thermal reservoirs,sources and channels formed by geothermal resources,and constructed a conceptual model of the geothermal system in this area.Based on the analysis of the geothermal system,this study divided the geothermal resources in the area,delineated the typical geothermal fields,evaluated the geothermal resources and the quality of the geothermal water,and proposed a target area for finding heat,which could provide scientific basis for the sustainable development and utilization of geothermal resources in Wanjiang Economic Belt and even Anhui Province.The specific research results are as follows:?1?This study systematically studied the development law of geothermal resources in the Wanjiang Economic Belt.The development of geothermal resources in this area is controlled by factors such as crustal thickness,regional geological structure,stratigraphic lithology,and fault structure.Geothermal resources in the area are mainly developed in structural units with thin crust thickness and high geothermal flow values,such as Luzong Basin,Dabie Mountain Uplift,Chaohu Dome Fault Fold Belt,etc.Hot water is mostly exposed at the intersection of northeast,near east-west heat control fault structure and northwest guide water fault structure control.?2?The heat sources of geothermal resources in the study area are mainly the heat conduction from the upper mantle inside the earth,as well as the sharp heat of rock radioactive elements and the residual heat of magmatic activity in areas with strong tectonic activities such as the Dabie Mountain uplift and the Luzong Basin.The heat flow value is between 33.56 m W/m2?156.42m W/m2?mean 79.20m W/m2?,and the geothermal gradient is 1.59?/100m?5.49?/100m?mean 3.41?/100m?.It shows a trend of high in the west and low in the east,high in the north and low in the south.According to the heating characteristics of the temperature measurement curve,it is divided into four types of ground temperature increasing types,including linear heating type,stable invariant type,jumping mutation type,and first rising and then falling type.The linear temperature-increasing type and the stable and invariant type heat transfer are in the form of heat conduction,and the jump mutation and the first-rising and then-decreasing-type heat transfer are in the form of heat convection.?3?The chemical types of hot water in the study area are varied and have obvious zoning.The structural chemistry types of Luzong Basin,Dingyuan Basin and Chaohu Dome Fault Fold Belt are SO₄-Ca?Mg and HCO₃-Ca?Mg;the structural chemistry types of Dabieshan uplift,Jiangnan platform uplift are HCO₃-Na,HCO₃?SO₄?Na;the water chemistry types of the Meso-Cenozoic basins such as Hefei Basin are Cl?SO₄-Na type and Cl-Na type.The upland mountain geothermal water-rock interaction is low and the TDS is low.The dissolution of feldspar minerals,carbonates,silicates,sulfates and other minerals mainly occurs along the water flow path.The geothermal water-rock interaction in sedimentary basins has increased,and the TDS is higher.The water-rock interaction is characterized by the dissolution of rock salt and ion exchange adsorption.?4?Four types of geothermal resources are developed in the study area,namely carbonate rocks,clastic rocks,magmatic rocks,and metamorphic rocks.The geothermal reservoir temperature of clastic rocks in the Hefei basin is 60??70?,and the geothermal reservoir temperature of carbonate rocks in Dingyuan basin is between 50??75?.The geothermal reservoir temperature of carbonate rock in Luzong basin is 75?100?,and the magmatic rock and metamorphic rock in Dabieshan uplift and Jiangnan platform uplift is 110??120?.The geothermal reservoir temperature of carbonate rocks in the Chaohu dome fault fold zone is between 60??100?.The geothermal resource depth is between 931.80?2641.69m,and the thermal storage circulation depth is between 961.80 m?2671.37m.The hot water in each geothermal resource area is mainly recharged by modern atmospheric precipitation.The elevation of the recharge area is 416.67?1183.33m.The clastic rock thermal storage in sedimentary basins is in a closed state,there is a small amount of"paleo-deposited water",and the underground cold water mixing is not obvious.The upland mountain heat storage of carbonate rock,magmatic rock and metamorphic rock heat storage is in a semi-open to open state,and the mixing effect of cold and hot water is strong.The mixing ratio of underground cold water is between 60%and 88%.?5?In this study,conceptual models for the genesis of two types of geothermal systems were constructed,including sedimentary basin-type geothermal systems and uplifted mountain-type geothermal systems.The thermal reservoirs?clastic salt rocks,carbonate rocks?of the geothermal system in sedimentary basins are continuously distributed and have thicker thermal insulation caps.After obtaining heat from the inside of the earth's crust through deep circulation under normal geothermal gradient,hot water surges along the rock strata mainly through thermal conduction,and is exposed through geothermal drilling to form geothermal resources.The upland mountain-type geothermal system thermal storage?carbonate rocks,granite rocks,metamorphic rocks?is developed in the fault zone.Atmospheric precipitation?or surface water?circulates deeper along the diversion fault to the deep fault where it meets.Driven by the deep high temperature and high pressure,the geothermal energy is carried by the fluid to the surface mainly in the form of thermal convection,and hot springs or artificial drill holes are formed at the intersection of the fractures to expose.?6?It is estimated that the total reserves of geothermal resources in the study area are 1.35×1015KJ,the recoverable amount is 2.63×1014KJ,and the recovered resource amount can reach 2.95×107W.The Dabie Mountain uplift,Chaohu dome fault fold zone can be used as the target areas for the development and utilization of geothermal resources.