Study On The Geothermal Geological Structure And Deep Geothermal Source Mechanism Of Xiong’an New Area Based On Comprehensive Geophysical Exploration

On April 1,2017,the Central Committee of the Communist Party of China and the State Council issued a notice to establish Hebei Xiong’an New Area.The China Geological Survey has put forward four major work objectives for the geological survey of the Xiongan New Area.Two of the important objectives are to build a world-class "transparent Xiong’an" and to build a global model for geothermal resource utilization.One of the important tasks is to investigate the fine geological structure and serve the development and utilization of urban underground space and geothermal resources in Xiong’an New Area,which plays an important role in the construction of the Xiong’an New Area.In addition,the rational development and utilization of geothermal resources is of great significance for the country to achieve the carbon peaking and carbon neutrality goals.In this paper,we take Xiong’an New Area as the research object,focusing from the regional tectonic framework to the local structural characteristics as the research idea.Through the collection of regional gravity data,acquisition of 2D long section of deep reflection seismic,magnetotelluric data and high-precision gravity data in the core area of Xiong’an New Area,we obtain the fine geological structure above Moho in Xiong’an New Area.Combined with comprehensive logging and geothermal logging data,we study the geophysical characteristics of Rongcheng geothermal reservoir by obtaining the velocity structure of Rongcheng high with the innovative multiscale full waveform inversion method.We study the deep heat source mechanism of Xiong’an New Area by comprehensive use of deep reflection seismic and magnetotelluric results,and study the geothermal system of Xiong’an New Area in combination with geothermal cap rock,geothermal reservoir,geothermal channel and water supply.The results of comprehensive geophysical exploration show that the Xiong’an New Area and its periphery have a tectonic pattern of "four convexes sandwiching three concaves and one slope".The middle part of piedmont fault of Taihang Mountain is characterized by "steep up and slow down".The overall trend is NE.The depth of the upper break point is about 400 meters,and the depth of the lower break point is about19 km.It does not cut through the lower crust and is not a deep fault.The activity of the middle part of piedmont fault of Taihang Mountain has been weak since Neogene.The depth of the Moho in the Xiong’an New Area is approximate flatten,with an average depth of about 34-35 km,showing the stable state of the regional structure.We firstly carry out 1:25000 high-precision gravity exploration in the core area of Xiong’an New Area.The results of the deep reflection seismic,magnetotelluric and highprecision gravity in the Xiong’an New Area show that all the seismic event,electrical structure and residual gravity anomalies are discontinuous in Eastern Rongcheng high and Western Rongcheng high.According to these evidences,the Rongcheng high is divided into two secondary structural units,the Rongxi high and the Rongdong high.Aiming at the difficulty of imaging carbonate fractured geothermal reservoir in Rongcheng high on seismic profile,we propose the idea of using full waveform inversion to obtain high-precision velocity structure inside the bedrock,and then directly interpret and delineate the scope of geothermal reservoirs from the fine velocity structure.In response to the problem of practical application of full-waveform inversion in conventional land seismic data,we develop a multi-scale full-waveform method in Laplace-Fourier domain based on layer stripping,propose a strategy of progressive inversion from shallow to deep,from near offset to far offset,from low frequency to high frequency.The strategy is successfully applied to the deep reflection seismic data in the Xiong’an New Area.The correctness of the result is verified by the methods of seismic record forward modeling,prestack depth migration,and logging data comparison.The resolution analysis is implemented with the checkerboard test.The reservoirs of Jixian System Wumishan Formation and Changcheng System Gaoyuzhuang Formation which are characterized with low velocity,low resistivity and low geothermal gradient are identified from the velocity profile.Their scope is delineated on the profile.We study the geophysical characteristics of the lower crust in the Xiong’an New Area and its periphery by using the combined method of deep reflection seismic and magnetotelluric.The results of magnetotelluric and deep reflection seismic have a high degree of agreement.The electrical characteristic of the lower crust is alternating high and low resistivity from northwest to southeast.The area with high resistivity corresponds to the approximate "reflection blank area" on the deep reflection seismic profile,and the area with low resistivity corresponds to the "dome-like" uplift of the reflection event on the deep reflection seismic profile.The contribution of the crustsource heat flow and mantle-source heat flow in the surface terrestrial heat flow of Niutuozhen high were quantitatively calculated by the "back-stripping” method.Among the surface terrestrial heat flow,the mantle source heat flow accounts for70.34%,and the crust source heat flow accounts for 29.66%.It is characterized with a typical "hot mantle cold crust" type.We build the deep geothermal geological model of“two factors” with the combination of the geothermal system elements and deep geothermal source mechanism.