Research On Lithospheric Electrical Structure And Deep Controlling Factors Of Geothermal Field In The Northern Songliao Basin

The Songliao Basin is located in the eastern part of the Central Asian Orogenic Belt.Affected by the superposition of multiple tectonic systems,the geothermal resources endowment.As a whole,the geothermal field has the characteristics of high in the middle and low in the surrounding rings.In recent years,a large number of geological and geophysical works have elucidated the material composition and structural characteristics of the Songliao Basin,and the geothermal research of shallow reservoirs is relatively high,but the thermal control elements in the deep part of the geothermal field are still unclear,the deep thermal driving mechanism and heat transfer model in the Songliao Basin need to be further studied.The lithosphere-scale three-dimensional electrical structure model is very important to improve the understanding of the geothermal field in the Songliao Basin.This paper mainly uses the broadband magnetotelluric sounding method to carry out area observations in the northern Songliao Basin,and obtains 107 broadband magnetotelluric measuring points,the distance between the measuring points is about20 km,and the distribution range is 123°E?127°E,45.5°N ?48.5°N.Using the phase tensor to conduct dimensional analysis,the results show that the ? value is less than 3when the period is 0.01?1 s,and the ? value is generally greater than 3 when the period is 1?1000 s,that is,the low-frequency phase tensor value is generally larger.It shows that the deep part of the study area has strong three-dimensionality,so the paper uses the nonlinear conjugate gradient method to carry out the 3D inversion.The model substitution method was used to verify the effective depth of the resistivity model,and the effective depth of the eastern electrical structure was 59 km.The western data is less sensitive,in some areas,the effective depth is more than 41 km,the Bostick depth approximation method was used to calculate the penetration depth of the two north-south profile data in the western part of the study area.It was found that the penetration depth of most MT data can reach more than 50 km,so the effective depth of the final model is confirmed to be 50 km.The model replacement verification was carried out for the main anomalies,which confirmed the reliability of the inversion results.The resistivity model shows good layering characteristics,namely high-conductivity sedimentary layers,discontinuous high-resistivity layers in the crust and high-conductivity layers in the upper mantle.The resistivity model shows 10 high conductors:(1)The high conductors C1,C2,C3,C4,C5,and C6 at a depth of 3 km in the shallow sedimentary layer are Cretaceous sedimentary layers,which can be used as shallow heat storage of different sizes,C7 is the magma chamber below Wudalianchi volcano;(2)The high-conductor C8 buried at a depth of 5 km in the upper crust corresponds to the Carboniferous-Permian strata deposited in the Late Paleozoic in the Songliao Basin,the high conductors C9 and C10 displayed at a depth of 7 km in the lower crust are interpreted as magma chambers formed at the bottom of the upper crust according to their occurrence locations;(3)The high conductors in the crust gradually connect with each other in the process of downward extension to form the upper mantle high conductivity layer,this high conductivity layer is believed to be formed by the combined action of aqueous fluids and partial melting of the rock.The conduction layer is formed by the combined action of the aqueous fluid and the partial melting of the rock.There are six crustal-scale high-resistivity bodies around the study area.It is believed that the high-resistivity bodies R1,R5 and R6 below the western slope area are granite intrusions,and the R3 below the central depression area may be a reflection of the Precambrian crystalline basement.The R2 and R4 existing in the southeast uplift area are manifestations of crystalline magmatic rocks.According to the three-dimensional resistivity model,combined with the measured heat flow data and drilling data,the paper identifies the deep heat control elements in the geothermal field of the basin,and establishes a geothermal system model:(1)The upwelling mantle thermal material is the main heat source of the basin,the magma chamber formed by the upwelling magmatic fluid at the bottom of the upper crust is the secondary heat source,and the radioactive heat generated by the intrusive granite mass is the local heat source,the three heat sources jointly transfer heat upward through the existing deep and large faults;(2)Affected by the upwardly conducted thermal material,a thermal reservoir is formed in the shallow part of the study area.Except for the shallow thermal reservoir in the sedimentary layer,the Precambrian crystalline basement R3 with huge thickness and high resistivity in the southern part of the central depression can be used as a deep thermal storage by virtue of the large amount of gneiss in it,which provides favorable conditions for geothermal preservation;(3)There is a low permeability mudstone caprock above the thermal reservoir.The central depression area in the study area is a large negative unit that has dominated the geological structure for a long time,the stratum is well-preserved,with no defects,and the deep part is a dual-reservoir model of heat storage,which has the best storage and preservation of thermal materials,so the geothermal anomaly in this area is the highest.In other areas of the study area,under the long-term denudation,the sedimentary strata are missing to varying degrees,and at the same time,there is a lack of crystalline substrates that accumulate heat,so that heat cannot be effectively preserved.