Research On The Structural Characteristics,Formation And Evolution Of Nanchong Structure In Central Sichuan Basin

Nanchong structure is located in the low and gentle fold region of central Sichuan basin,which is located in the large slope of central Sichuan basin uplift.For a long time,the former people's research on the Nanchong structure in the central Sichuan basin,most of them are oil and gas reservoir evaluation studies in the Xujiahe formation or the Jurassic strata,lack of tectonic geology research about Nanchong structure.In addition,previous research have rarely analyzed the deep structural characteristics,tectonic kinematics and tectonic dynamics of Nanchong structure through 2D seismic section and section inversion.This paper mainly uses 2D seismic line profile and related drilling data in the research area as the main research method.By identifying and interpreting seismic reflector of seismic section,identifying and analyzing unconformable surface features in the research area and restoring balanced section inversion,analyze the geometrical characteristics,kinematics of Nanchong structure and the power sources of the formation of Nanchong structure.Finally,verification was carried out by constructing a physical simulation experiment.The research has obtained the understanding of the actual regional geological evolution background and regional structural deformation characteristics.The main achievements are as follows:(1)This paper analyzes the development of unconformity in the Sichuan basin,combined with the identification and interpretation of the unconformity surface of the relevant seismic line profile in the Nanchong tectonic research area,the sedimentary caprock of the Nanchong structural research area is divided into four tectonic stratigraphic sequences: Sinian tectonic sequence,the lower Paleozoic tectonic sequence of the Cambrian-Silu,Permian-middle triassic tectonic sequence and upper Triassic-Jurassic tectonic sequence.(2)Research the geometric characteristics of Nanchong structure,The Nanchong structure is located at the slope of the uplift in the central part of the Sichuan basin,the buried depth of the sinian strata increases in the north-west and north-east directions.The research area was affected by the Caledonian movement,the Silurian strata and the upper Ordovician strata are severely denuded.In some areas,the middle and lower Ordovician strata are unconformably in contact with the overlying lower Permian strata.the stratigraphic deposits in the deep and lower tectonic layers of the research area are relatively stable,the thickness of the formation varies little.The middle structure layer is bounded by the strata of the fifth section of Jialingjiang formation,the fifth member of the Jialingjiang formation and the Leikoupo formation are thickened in the core of the Nanchong structure.The thickness of stratigraphic strata in the upper Permian strata and the lower triassic strata below the fifth member of the Jialingjiang formation have not changed much,less affected by structural deformation.The thickness of upper Triassic Xujiahe formation strata and the middle and lower Jurassic strata is stable.In general,the Nanchong structure belongs to the decollement fold.the southeastern section of the Nanchong structure has a bifurcation phenomenon on the surface,more considerations are caused by topographical erosion,not by tectonic movements.(3)Research the kinematics characteristics of Nanchong structure,according to the results of the shortening of the balance profile of the four lines 01,02,03,and 04 passing through the Nanchong structure,the fifth section of the Jialingjiang formation was used as the main slip surface,the average shortening amount of strata above the slip surface in the north-western section of Nanchong structure is 144.27 m,the average shortening amount of strata above the slip surface in the middle section is 333.22 m and the average reduction of the formation above the slip surface in the eastern section is 27.51 m.In general,the north-western section of the Nanchong structure and the middle section are strongly affected by tectonic stress and deep fold morphology is relatively developed.The south-eastern section of the Nanchong structure is less affected by tectonic stress and deep fold morphology is less developed.(4)Research the dynamic characteristics of Nanchong structure,The Nanchong structure was formed in the late Yanshan movement and finally finalized in the Himalayan period.By designing and implementing a structural physics simulation verification experiment,according to the physical simulation experiment process observation and the experimental model slice results,the possibility of forming the Nanchong structure under the uplift tectonic stress is further verified.