Study On The Spatial Structure Of Deep Thermal Storage And The Process Of Hydrothermal Differentiation In Xiong'an New Area

Xiong'an new area is located in the central part of North China Plain,where geothermal resources are widely distributed and shallowly buried with high temperature,large reserves and excellent water quality.However,the occurrence conditions of deep geothermal resources in this area are complex.At present,the structure and genetic mechanism of deep karst thermal reservoir are unclear.Geothermal scientific drilling provides a direct means to obtain the data related to the spatial structure of deep thermal reservoir,and it also provides an opportunity to obtain the deep fluid samples.Geothermal fluid carries huge deep information,leaving imprints on hydrochemical,isotopic and microbial special compositions.They can be used to identify the formation and evolution mechanism of geothermal fluid.Study on deep thermal storage spatial structure and hydrothermal differentiation process in xiong'an new areais of great scientific significance to the improvement of the understanding and the rational development and utilization of the deep geothermal resources in the future.In this study,geothermal scientific drilling,field investigation,laboratory experiments,hydrochemistry,isotope technology,microdrop digital PCR technology and high-throughput sequencing technology were used to study the spatial structure and hydrothermal differentiation process of deep thermal reservoir in xiong'an new area,to analyze the deep thermal storage spatial structure,the hydrochemistry and isotopic composition characteristics of geothermal fluid,and to explore geothermal indicative significance of microbial community characteristics.Based on the study above,the conceptual model of the genesis mechanism of geothermal energy in the study area was established.The main conclusions were as follows:Xiong'an new area belonged to the eastern North China Craton and the middle of Jizhong depression.The destruction of the North China Craton made the lithosphere thin and the crust thin,so the mantle was the main heat source in the study area.There were many secondary structural units with abundant fault structures,which played an obvious role in controlling local structures.They were good heat conduction and water diversion channels.The deep heat flowed upwelling through the fault and formed hydrothermal convection with the heat flow in the heat reservoir,resulting in thermal anomaly.From the drilling data,the formation lithology and the spatial structure of the thermal reservoir were obtained.The heat storage was shallow in the uplift area and deep in the depression area,indicating that the heat was concentrated in the shallow part of the uplift area and in the deep part of the depression area.The geothermal gradient in the study area was generally 3.0⁸.0?/100m.Vertically,the quaternary system was generally 2³?/100m,the Neogene system was 3.0³.7?/100m,and the top of the bedrock was3.09⁴.15?/100m.The geothermal gradient within the bedrock was reduced to 1.8².4?/100m because of the high thermal conductivity of dolomite.Meanwhile,the distribution of terrestrial heat flow value in the study area was from 55 to 125mW/m²,which was characterized by high heat flow value in the uplift area and low heat flow value in the depression area.According to the analysis of hydrochemical and isotope data,the hydrochemical type of shallow water in the western mountainous area of Baoding was HCO₃-Ca·Mg water,while the hydrochemical type of deep geothermal water in the study area was Cl·HCO₃-Na water or Cl-Na water.The excess HCO₃^-mainly originated from the mixture of biogenic CO₂ and abiogenic CO₂.The deep geothermal water was in a relatively closed environment where strong sulfate reduction existed.The ratio of?Na/?Cl was 1.10-2.39,the ratio of Cl/Br was 409.29-735.29,the ratio of most desulfurization coefficients 100??SO₄/?Cl was less than 1,the ratio of salinization coefficient?Cl/??HCO₃+?CO₃?was 0.76-3.31,the ratio of?Cl/?Ca was 6.33-33.03,and the geothermal water belonged to continental leaching metamorphic water.The main recharge source of deep geothermal water was atmospheric precipitation,and the recharge area was Taihang Mountain Area in the west of Baoding.The age of deep geothermal water was more than 20000years,and it was in the regional hydrologic cycle with slow water alternation.A new technique ddPCR for the detection of sulfate-reducing bacteria in deep geothermal water was developped.The technique was used to detect sulfate-reducing bacteria in deep geothermal water,shallow water and soil samples collected from Xiong'an new area with the average contents of 4.0×10³±8.4×10³copies/ml,1.6×10²±3.5×10² copies/ml and 1.5×10³±1.2×10³ copies/g-dw,respectively,which showed that the deep geothermal water was rich in sulfate-reducing bacteria.A new generation of high-throughput sequencing technology detected 38 phylum and 541 genera in deep geothermal water in Xiong'an new area.Most of the dominant genera in the deep geothermal water were sulfate-reducing bacteria,such as thermodesulfovibrio,thermodesulfobacterium,thermoanaerobacteraceae,desulfovirgula and desulfotomaculum.The results of functional gene prediction showed that the microbial carbon fixation was very strong and sulfate reduction was relative strong.However,the microbial methanogenesis was very weak,which meant that the majority of CH₄ in the deep geothermalwater originated from abiogenic CH₄.Under the condition of large flow pumping,there were two main types of bacteria in deep geothermal water,Firmicutes type and Proteobacteria type,and the latter tended to shallow water specific Proteobacteria type which indicated that the deep geothermal water was recharged from the shallow water.The dominant genera of Proteobacteria in the deep geothermal water was different from those of Proteobacteria in the shallow water above the borehole.The source of shallow water recharge for the deep geothermal water was not from the vertical recharge of the shallow water above the borehole,but from the lateral recharge of the shallow water obtained the thermal energy through the deep circulation,or from the shallow water recharge through the connection of the fault zone.The deep geothermal water resources had the renewability of resources.Finally,the model of geothermal energy accumulation mechanism in the study area was established based on geothermal geology,regional structure,spatial structure of thermal reservoir,and the indicative significance of geothermal energy accumulation mechanism from characteristics of hydrochemistry,isotopes and microbial communities of the geothermal fluid in the study area.