Geometry And Evolution Of Changning Anticline In Southern Sichuan

Changning anticline is located in the changing area on the southern edge of Sichuan Basin,which is a favorable block for shale gas exploration and development in China.However,there is no consensus on the formation,evolution and genetic mechanism of Changning anticline.In order to understand the structural characteristics,formation and evolution of Changning anticline,and provide reference for shale gas exploration in this area,this paper comprehensively uses regional geological data,twodimensional seismic profile and drilling data,interprets the surface and deep structures of Changning anticline.The geometric characteristics of Changning anticline are analyzed in depth.The structural evolution process is analyzed by using balanced profile technology,and the dynamic mechanism of Changning anticline is studied through structural physical simulation.We get the following understanding:(1)Changning anticline is an asymmetric anticline with NW-SE strike.The SW wing is gentle(10-20°),and the NE wing is steep(30-40°).The NW end of the anticline bends southwestward and forms drag fold,which is related to the strike slip of the southern Huayingshan fault zone.The 2D seismic profile shows that large thrust faults are developed in the basement of Changning anticline,extending into the basin in a step shape,and the dip angle of fault slope is about 13°.Combined with the structural style and the theory of fault related fold,the Changning anticline is fault-bend fold,and its structural form is closely related to the basement fault.(2)The strata in the study area are well developed.The core of Changning anticline is severely eroded and exposed to Cambrian and Ordovician.The two wings of the anticline mainly expose Silurian,Permian,Triassic and Jurassic,Devonian and Carboniferous are absent.Combined with the development characteristics of strata and relevant drilling data,there are mainly many sets of detachment layers,such as Middle Cambrian gypsum rock,Lower Silurian shale and middle lower Triassic shale and gypsum rock.(3)The analysis of tectonic evolution shows that the formation and evolution of Changning anticline is closely related to Himalayan tectonic movement.In the early Himalayan period,the basement of Changning anticline developed step like buried faults under the action of south-west compressional stress,which led to the overall folding deformation of overlying strata and the formation of fault-bend fold.In the late Himalayan period,the Changning anticline was rapidly uplifted and subjected to strong denudation,resulting in the formation of the present structural features.(4)The dynamic mechanism of Changning anticline is further analyzed through structural physical simulation experiments based on the study of structural geometry and kinematics characteristics of Changning anticline.The experimental results show that: influenced by the long-distance effect caused by India sub plate collision,the basement of Changning anticline develops step like concealed faults,and controls the structural morphology,formation and evolution of Changning anticline.