Study On Structural Characteristics And Deformation Mechanism Of The Shuangyushi Area In Northwestern Sichuan

In recent years,significant progress has been made in oil and gas exploration in Paleozoic strata of northwestern Sichuan Basin.Shuangyushi(SYS)area is located in the western and northern part of sichuan province,which belongs to the Piedmont with complex structure.And this area has experienced multi-periods and multi-cycle tectonic movements during the formation and evolution process,which leads to a variety of unconfirmed schemes for structure interpretation.Meanwhile,further exploration of oil and gas exploration is also restricted.Therefore,it is necessary to systematically analyze the structural geometric characteristics,tectonic evolution characteristics and deformation mechanism of the SYS area in the northwestern Sichuan Basin.With the help of interpretation on two-dimensional seismic section and analysis of drilling data in the research area,combining with balanced cross-section restoration and structural physical simulation,this paper demonstrates the research on structural characteristics and formation evolution of the SYS,and the analysis for the deformation mechanism of concealed structural belt.In additional,we attained the recognition of deformation characteristics which match the structure of actual area.The achievements are as follows:(1)The structural framework of the SYS area features zoning and layering.Transversely,the thrust structure belt and the concealed structural belt is bounded by the NE-SW trending No.1 fault.Longitudinally,the gypsum rock strata of Upper Triassic Feixianguan Formation and the mud shale formation on the bottom of Cambrian are regional detachment layers,which play a controlling role in the structural deformation of the area.In additional,Three deformed layers which is lower,middle and upper,were divided by the two detachment layers.The Precambrian strata are the lower deformed layer with weak deformation and fault rarely developed.The Cambrian-Lower Triassic Jialingjiang Formation is medium-deformed layer with strong deformation,and it is characterized by the development of thrust structures and structural deltas.The strata above the Middle Triassic Leikoupo Formation are upper deformed layer,which are characterized by passive deformation.(2)According to the balanced cross-section,we believe that the concealed tectonic zone in the SYS area is the result of multistage tectonic movements.During the Hercynian period,some small synsedimentary normal faults developed under the regional extensional environment.In Late Indosinian epoch,the existent normal faults reversed and the rudiment of pop up structure formed because the dynamic environment changed into compressive environment.During the Himalayan epoch,the previous structures were superimposed and reformed under compression,and finally the present structures formed.(3)Through structural physical simulation experiments,combined with the balanced cross-section,we consider that basic structural pattern of the concealed tectonic belt in the SYS area formed in Late Indosinian.The Songpan-Ganzi Plateau was uplifted,therefore,the original pop up structure in the SYS area formed under the NW-SE compression which was caused by the gravity sliding.Later,in the Himalayan period,due to the subduction of the Indian plate to the Eurasian plate,the pre-structures were transformed under NW-SE compression and finally the present structural pattern formed.