Study On The Current In-situ Stress State Of The Songliao Basin

Songliao Basin is the largest continental lacustrine sedimentary system of Cretaceous that has been discovered in the world,and it is also one of the regions with the most complete continental Cretaceous stratigraphy and rich in oil resources.With the discovery of commercial oil and gas in the deep fault depression and the occurrence of high frequency seismic activity in the basin in recent years,the study of geodynamics in this area has gradually attracted people's attention.The magnitude and direction information of the in-situ stress in the deep crust is closely related to the study of the mechanism of earthquake geological hazards,mineral resources exploitation,and the development of underground space.It is an important basic parameter of geodynamic research.On the basis of in-depth understanding of the regional geological background,fault structure distribution,and layered structure of the crust in the Songliao Basin and adjacent areas,a detailed study of the current in-situ stress environment and its distribution characteristics in this area is of great significance for understanding the geodynamic and the influence of deep fault activities on the stress field in this area.Based on the systematic collection and analysis of regional geology,crust and lithosphere dynamic characteristics,block division,regional tectonic stress field and seismic activity in Songliao Basin and its adjacent areas,this paper first focuses on the analysis of in-situ stress measurement results by ASR(anelastic strain recovery)method in deep SK-2 borehole,and obtains the rule of current in-situ stress variation with depth of sedimentary cover and basement in deep Songliao Basin(6-7km).Secondly,according to the characteristics of crustal structure,structural division and distribution of deep and large faults in the study area,a three-dimensional geological model of Songliao Basin and its adjacent area is established.Based on the linear elastic finite element numerical simulation method,the current three-dimensional geological model of Songliao Basin and its adjacent area is developed by using the general simulation software ANSYS.Using the measured in-situ stress and focal mechanism solution as the boundary constraints condition on numerical simulation of stress field.The current three-dimensional tectonic stress field numerical simulation study was carried out in the Songliao Basin and its adjacent areas.We obtained the magnitude and direction information of horizontal principal stress,and elastic strain energy density with depth in Songliao Basin and its adjacent areas.The influence of deep fault zone on stress field characteristics in the study area is discussed,and the reasons for the formation of current stress field in Songliao basin are analyzed.Through the study of the present geostress environment in the Songliao Basin,the following conclusions and understandings are obtained:(1)The anelastic strain recovery(ASR)method was employed to measure the stress state in the SK-2borehole in the Songliao Basin,Northeast China and the variation law of current in-situ stress with depth of sedimentary cover and basement in deep Songliao Basin(6?7 km)is obtained.In the sedimentary cover(6296?6335 m),the maximum principal stress?₁is nearly vertical,which is in the normal faulting stress regime dominated by gravitation,in accordance with the normal faults observed on seismic reflection profiles within the sedimentary cover.In contrast,the basement(6646?6846 m)shows that?₁ is nearly horizontal,suggesting a strike-slip or reverse-faulting regime.The stress state consistent with the focal mechanisms of earthquakes(7?15 km)in the vicinity of ICDP SK-?borehole.The results show that the stress state differs significantly between the sedimentary cover and basement.The difference of stress state may imply that the far-field stress generated by the western Pacific plate has limited effects on the sedimentary cover,and the normal stress environment of the fault depression period is retained.The present stress state of the basement shows that it is closely related to the present tectonic movement of the subduction of the Western Pacific plate.(2)Through the numerical simulation of the three-dimensional tectonic stress field,it is found that the maximum,minimum horizontal principal and vertical stress in the Songliao Basin and adjacent areas within the crustal depth range of 0?35 km is 17.20?1027.00 MPa,13.00?994.00 MPa,7.83?1130.00 MPa respectively.The three principal stresses basically increase linearly with the increase of depth.The relationship between the three principal stresses is:?_v>?_H>?_h in the depth range of 0?7 km,which belongs to normal fault stress state and consistent with the measured value;?_H>?_v>?_h in the depth range of 7?35km,showing a strike-slip stress state,which is consistent with the stress state inferred by the focal mechanism solution of shallow earthquakes in Songliao basin.In the Songliao Basin and adjacent areas,there is little difference in the azimuth of the maximum principal stress in the crust depth between the shallow and deep crust,the dominant orientation in most tectonic units is is NE?NEE.Except for the northern margin of the North China block and some parts of the Xing'an block,which the principal compressive stress azimuth is NWW.In each sub block,the distribution of principal stress in each structural unit of Songliao Basin and its adjacent area is different in both horizontal and vertical directions.The distribution of principal stress in stable sub blocks is similar,showing that the principal stress tends to be stable in the same depth range.(3)The subduction direction of the Western Pacific plate as the dynamic boundary condition,studying the relationship between deep faults and in-situ stress characteristics obtained by numerical simulations.It is considered that the Western Pacific plate and Yilan Yitong fault and Dunhua Mishan fault in the northern part of Tanlu fault zone have a certain influence on the formation of the present stress field in Songliao basin.The compression produced by the NWW subduction of the western Pacific plate acts on the NE-trending Tan-Lu fault zone,and its shear component and compression component cause the right-lateral strike-slip and thrust of the Tan-Lu fault zone.Therefore,the formation of the present stress field in Songliao basin may be caused by the compression of the NWW subduction of the Western Pacific plate to the Eurasian plate and the dextral strike slip of the northern segment of the Tan-Lu fault zone.