Geophysical Field And Structural Characteristics Of The Basin Group On The West Of Songliao Basin

The study area is located in China's HeiLongJiang province and Inner Mongolia autonomous region of china,west of SongLiao basin,and is 1650 km long from the north to the south.There are many faulted basins in the study area,which are mainly composed of Mesozoic sedimentation.They are referred to as “western basin group” and are important oil and gas exploration and development bases in Northeast China.As a basin group,affected by regional and deep structure,it has common effects in structural changes,stratigraphic changes,hydrocarbon generation environment and other aspects,and also causes different manifestations of each basin due to different macroscopic tectonic conditions.So far,there are few unified studies on the west basin of SongLiao basin.Through the comprehensive geological-geophysical research,the paper plans to make new achievements in the controlled regional tectonic stress field and deep structure,structural characteristics and properties of the basin group,and oil and gas condition analysis of the Paleozoic strata in the study area.In the western basin group of SongLiao basin,an 1800 km long magnetotelluric profile was firstly organized and implemented.A total of 65 broadband magnetotelluric sounding points and 33 deep points on the long period magnetotelluric side were collected.In this paper,nonlinear conjugate gradient inversion was performed on the data to obtain the two-dimensional electrical structure model.The conclusions are as follows: the inhomogeneity of the lithospheric thickness in western basin group is controlled by different dynamic backgrounds;the dichotomy of the Erlian Basin explains the combined action of the closure of the Paleo-Asian ocean and the closure of the Mongolian-Okhotsk ocean in this region;the northern section of the section,represented by the Hailar basin,labrador basin and mohe basin,has the characteristics of thin lithosphere and thick asthenosphere.The structure of the lower part of the greater Hinggan mountains and Hailar basin indicates the process of transforming the lithosphere by the Cenozoic magmatic activity,which explains that the northern section of the study area may be affected by the subduction of the Pacific plate,or it may be the reflection of the deep magmatic activity and the vertical interaction between crust and mantle.The velocity structures of P and S waves in the crust of northeast China are obtained by seismic tomography in this paper.The imaging results show that the Pacific plate continues to subduct westward,and the leading edge of the subduction plate stagnates in the mantle transition zone in the east of the greater Hinggan lingtaihang mountains gravity gradient belt.We believe that due to the large-scale westward subduction of the Pacific plate,complex geodynamic processes such as plate dehydration and low-velocity thermal material upwelling occur in the mantle wedge.The leading edge of subduction plate drives the uneven distribution of mantle flow in the upper mantle to strongly act on the upper lithosphere,which has an important influence on the formation and evolution of the deep crust-mantle structure and even the gravity gradient belt of the greater Hinggan mountains.And the threedimensional density structure of upper mantle in the study area was obtained by using the the large-area gravity data inversion including the northeastern region,parts of North China and the Sea of Japan.Comparing it with the three-dimensional velocity structure obtained natural earthquake to get the interpretation,the results show that the subduction of the Pacific plate at the Japan trench in high density anomaly,and lateral expansion to the west,the depth direction has increasing trend;Horizontal high density bodies were also found in the lower transition zone of the greater Hinggan mountains fault zone,which is presumably the result of mantle lithosphere devolution under the greater Hinggan mountains fault zone.Based on the study of the gravity anomaly characteristic,the electrical method in the interpretation section and other geophysical data and geological data,the late Paleozoic in Erlian basin and its neighboring area potential hydrocarbon source rock to describe,analyze,predict its preliminary late Paleozoic strata depth distribution roughly within 3km to 5 km,and it has two distribution centers with thicknesses ranging from 1000 m to 2000 m.The correspondence between its overall distribution characteristics and Erlian basin boundary is compatible,the widely distributed late Paleozoic hydrocarbon source rocks can provide a new direction and new ideas for oil and gas exploration of Erlian basin.From Mohe basin,Hailar basin to Erlian basin,the study area constitutes the nappe tectonic system of the west basin group.Based on the systematic study of five nappe structures in three basins,the nappe directions are preliminarily divided into two types: the first type of nappe is from northwest to southeast,which is obvious in three basins;the second type of nappe is from southeast to northwest,which is shown in Hailar basin and Erlian basin.The first type of nappe is derived from the closure of the Mongolian-Okhotsk Ocean from west to east.The second type of nappe is derived from the long-range stress of subduction of the Pacific plate.