Genesis And Rational Development Of Typical Geothermal Field In The Songliao Basin

Geothermal energy is characteristic of large reserves,wide distributon and stable supply.It is a practical and feasible renewable energy.To push the development and utilization of geothermal energy is of great strategic significance for the adjustment of national energy structure and the realization of carbon neutralization.The Songliao Basin is not only a typical rift basin,but also the largest and hottest continental sedimentary basin in China.It is abundant in geothermal resources.Since the structural conditions is unique,the thermal water have diverse origins,distributes unevenly and the genesis is complex.Lindian geothermal field,located in the sedimentary center of the northern Songliao Basin,is a famous"hometown of hot springs".With the long-term development of thermal water,the reservoir pressure decreases significantly.Therefore,to establish the genesis mechanism of Lindian geothermal field and formulating the rational development and utilization scheme of the thermal water have important theoretical and practical significance for improving the genesis model of geothermal system in rift basin and promoting the scientific development of geothermal resources in local and similar geothermal systems,and will play an important role in promoting the proportion of geothermal resources in clean energy production and consumption in the northeast cold region.This research is supported by the Heilongjiang provincial financial fund project"Pre-feasibility Exploration of Geothermal Resources and Geothermal Energy Heat Exchange Experiment in Nanyuan Development Zone,Qiqihar City,Heilongjiang Province(SRKC-2019002)".Through data collection and analysis,geothermal geological investigation and sample collection and tests,and using comprehensive technical methods including hydrodynamics,hydrochemistry,isotope and numerical simulation,this paper studies the genesis mechanism of Lindian geothermal field from four aspects:heat(geo-temperature field and heat source conditions),reservoir(geogeothermal waterand caprock conditions),source(fluid characteristics,genesis and water age)and runoff channel,summarizes the genesis model of geotherral system in rift basin,puts forward the rational development and utilization scheme of thermal water.The main findings and conclusions of this research are summarized as follows:(1)The characteristics of reservoirs and caprocks are obtained.The geothermal reservoirs in the study area are mainly consists of the Lower Cretaceous,i.e.the second and third members of Yaojia Formation,the second and third members of Qingshankou Formation and the third and fourth members of Quantou Formation.The buried depths of the reservoirs are 980?1670 m,1360?2080 m and 1500?2430 m,It has obvious heterogeneity in horizontal and vertical directions.The caprock cmoprises Quaternary,Paleogene and Cretaceous Mingshui Formation,Sifangtai Formation and Nenjiang Formation,with a total thickness of about 1220?2100 m,the heat insulation performance withrepect to conduction and convection heat transfer of the caprocks was evaluated to be good.The reservoir temperature is 47.4?89.0?,It belongs to medium-low temperature geothermal field.(2)The characteristics of the geo-temperature field,influencing factors and heat sources are revealed.The geo-temperature field in Lindian is characteristics of high thermal gradient and high heat flow.The thermal gradient is generally 3.0?4.3?/100m,and the heat flow is 66.4?78.6 m W/m~2.The formation of the geo-temperature field is controlled by deep and shallow geological factors.The deep geological factors are the Moho interface,the Curie interface uplift and the existence of low resistivity and high conductivity layer.The buried depth of the Moho interface in the area is less than 30 km,while the Curie interface is between 21?26 km,and the low resistance and high conductivity layer is about 18 km,with a the thickness of 7?10km.The heat source is mainly mantle heat conduction,followed by radioactive heat generation in the crust.The heat flow ratio between crust and mantle is 0.78?0.98,showing that the thermal structure of lithosphere is of a typical“hot mantle and cold crust”type.(3)The characteristics,genesis of geothermal fluid and water age are revealed.Based on the methods of hydrochemistry,environmental isotopes and petrochemistry,the identification system of thermal water sources in sedimentary basin is established.It is concluded that the water source in Lindian geothernal field is mainly atmospheric precipitation,and there is a small amount of water released from the aquitards during pumping,but no sedimentary water nor mudstone compaction water;The hydrochemical facies of different structural units are controlled by the hydrodynamic field,and change regularly from the recharge area to the discharge area;The thermal water circulates slowly and the ages is older than 43500 a.B.P.(4)Thermal water runoff channels were identified.Based on the principle of Neotectonic heat control theory,the water conduction and heat conduction properties of three main deep faults in the geothermal field are identified by the analysis of tectonic stress field,hydrochemistry and isotope field and geo-temperature field.The three faults are tensile shear faults,of which the Lindian Fault cuts to the top of Qingshankou Formation,connects the main reservoirs and plays the role of heat conduction and water trasport;The Lindian East Fault and Heiyupao Depression West Bourdary Fault cut to the bottom of the Jurassic and Quantou Formation respectively,but they are accompanied by a large number of secondary faults,which also have a certain role of heat conduction and water trasport.(5)The genesis model of geothermal system in rift basin represented by Lindian geothermal field is put forward.Based on study of reservoir,heat,water sources and runoff pathway of Lindian geothermal field,the typical genesis model of gothermal system in rift basin is proposed as“tectonic-thermal evolution controll the formation of rift basin,and the accompaning tectonic-sedimentary process controll water/heat origin and transport”.This genesis model is promising to provide a scientific support for exploration and rational development and utilizitin of geothermal resources in rift basins(6)The rational development and utilization scheme of the geothermal resources in the main exploitation area of Lindian county is proposed.The geothermal doublet operation should be adopted in Lindian geothermal field.The reinjection water is mainly the tail water after heat exchange.The results of Thermo-hydro coupled modeling shows that when the reinjection ratio is 75%,the pressure of the production well remains unchanged.In this senario,two reinjection wells are needed for one production well,and the allowable withdraw of a single well is 40 m~3/h,while the reinjection rate is 15 m~3/h.The regional allowable withdrawal is 6.56 million m~3/a.The reasonable well spacing is 400 m and 370 m for reinjection water whose temperature is25?and 35?,respectively.