| The exploration researches in recent years reveal that the Xujiaweizi(XJWZ)fault depression is an important gas fault depression.It is located in the northern Songliao Basin and has a good prospect for natural gas exploration.However,the XJWZ fault depression has experienced multiple tectonic movements,so the faults and the structural styles are complex.The tectonic evolution does not have a clear understanding and the control of faults and structures on the gas accumulation is not clear.These problems limit the exploration in the region.Therefore,it is necessary to in-depth analyze the structural geometric features and the structural evolution characteristics of the XJWZ depression.In order to get a better understanding of the structural characteristics in the XJWZ fault depression,this work started from the viewpoint of construction and reconstruction,based on the drilling data and seismic profile interpretation,combining with basin dynamics,tectonic geology and sedimentary geology,using balanced section technique and structural physical modeling.And it made a deep and systematic study of the fault system,structure styles and tectonic evolution of the XJWZ fault depression.It will have a solid foundation for the next oil and gas exploration.Finally,the research has achieved the tectonic geological understanding which conforms to the actual area,including the following points:(1)The XJWZ fault depression is a dustpan fault with faults on the west and overlaps on the east which developed in extension.Its structural framework features N–S blocking,W–E zoning,and depression alternating with uplift.There are four faults in the XJWZ fault depression.As a boundary controlling fault,the Xuxi fault has been identified as the main controlling fault for the formation and evolution of the XJWZ fault depression.The Xuzhong fault,the Songxi fault and the Xudong fault are the companion faults that developed during the formation of the depression.They exert little control on the overall structural pattern of the fault depression but make the structures in the fault depression more complex.The plane distribution of the faults in XJWZ fault depression is controlled by the Xuxi fault,while the distribution characteristics of the secondary faults are irregular and complex.But overall,there are many combinations of the faults in plane,such as En-echelon combination,“X” type combination as well as mist shape combination.(2)From the viewpoint of construction and reconstruction,through the fine interpretation of 3D seismic data,four stages of construction and four stages of reconstruction have been identified.Three structural styles developed correspondingly,including extensional structural style,extrusion structural style and reversal structural style.Extensional structural styles,such as fault terrace,Y type combination,horst-graben structure and normal fault related fold,mainly developed in the construction period.Extrusion structural style and reversal structural style mainly developed in the reconstruction period.Pop-up structure and reverse fault related fold are the typical extrusion structural style,and reversal structural style is a typical positive inversion structure.(3)The tectonic evolution process of the XJWZ fault depression can be divided into five stages,began with basement formation;second by the stage of initial rifting;followed by strongly rifting;then compression folding and finally by another stage of stable deposition.The basement formed before the Middle Jurassic.Initial rifting occurred in Huoshiling Formation sedimentary period.In this period,normal fault related fold developed under extensional environment.The prototypes of Xuxi sag,Xuzhong uplift and Xudong sag have been created.Strong rifting occurred in Shahezi Formation sedimentary period,horst-graben structure and fault terrace developed under the effect of extension.Compression folding occurred in Yingcheng Formation sedimentary period;thrusting structure and pop up structure developed under compressive tension,part of strata suffered denudation.In the late stage of Yingcheng Formation sedimentary,positive inversion structure developed.Stable deposition occurred after Yingcheng Formation sedimentary.The tectonic activity was weak,so that the structure deformation and the structure pattern were less influenced.(4)The results of structural physical modeling and the balanced cross section show that the formation and evolution of the XJWZ fault depression were mainly controlled by the western sag-controlling fault,Xuxi fault.The structural pattern of blocking in the N–S was due to the change in the texture and the occurrence of the Xuxi sag-controlling fault.The basic structural pattern of the XJWZ fault depression formed in a unified extensional tectonic dynamics environment.The Xuxi sag,Xuzhong uplift and Xudong sag were the results of the same tectonic dynamics.The lateral extrusion only took a role in the transformation of the Xuzhong uplift,which made the Xuzhong uplift and the structural pattern of the fault depression more complex.Normal fault related fold developed in the construction period of the XJWZ fault depression,and the anticline developed into the Xuzhong uplift,the synclines developed into the Xuxi sag and Xudong sag.In the reconstruction period,under the effect of the pre-existing faults,reverse fault related fold developed in the XJWZ fault depression.In compression,the strata above the ramp thrust to the flat and folded under the controlling of the fault plane.The positive inversion structural style developed in the XJWZ fault depression due to the superimposition of extension in the construction period and compression in the reconstruction period.The complex structural pattern could form in the same tectonic dynamics environment owing to variation of the structural boundary conditions.The mechanism is not necessarily complicated and the complex structure may form in the same tectonic movement period. |
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