Tectonic Characteristics And Sedimentary Evolution Of The South Terrace In Gaoyou Sag

Comprehensively utilizing data of geology,well-logging and seismic and so on,this paper finely interprets the 3D structure of the south terrace of Gaoyou sag in Subei basin.On the basis of the analysis of the development of basement faults in the research area,we focus on the geometrical features and kinematic characteristics of Wubao terrace and Zhenwu terrace,define the genetic mechanism of terrace and the mode of tectonic development,and analysis the internal tectonic divergence.The types and characteristics of sedimentary facies in south terrace were discussed,and also the distribution of sedimentary evolution after single well phases and cross-sectional phases according to the data of cores and well logging.At last we conclude the correlations between the structure,sedimentary features and hydrocarbon accumulation.Diversified data such as deep seismic data indicate that Zhenwu Fault is a key basement fault in Gaoyou Sag.It develops in linear bending overall.The east part strike to NE and incline to NW,while the west part strike to NEE and incline to NNW.It is a spade-like reverse fault,which is controlled by the tectonic compressional stress field during the period of Mesozoic in the eastern China,including three evolutionary stage which are the early Indocinia,the late Indocinian and the Yanshan.The geometric research of the south terrace indicates that the trend of eastern part of Wubao terrace is NE,while that of western part of Zhenwu terrace is NEE.Wu?1?fault and Zhen?1?fault are level-I boundary fault which control the development of depression,while Wu?2?fault and Zhen?2?fault are level-II fault that control the terrace.The planar combined types of faults in the south terrace are skew-type,en echelon parallel-aligned,grid-type.The section combined types of faults are simple half graben,rolled half graben and complex half graben,Wubao and Zhenwu terrace are different in the characteristics of geometry.On the base of establishment of stress-strain ellipse model,we calculated extension and extensional ratio along profiles in different direction in all periods.The conclusion reveals that the advantageous extended orientation in Gaoyou Sag has experienced the process of NW?K₂t₂-E₂d??NS?E₂s??NW?Ny-Qd?since late Cretaceous period,and it provides in-depth understanding of the direction of tectonic stress field since then from a point of quantitative view.We calculated the average slip rate of main faults in terrace.The results show that Wu?1?fault,Zhen?1?fault both started in the period of K₂t₂+E₁f_1,peaked in the period of E₁f_4,weakened apparently in the period of E₂s,stopped in the period of Ny.Wu?2?Fault and Zhen?2?Fault started slightly later than the first two.Secondary faults formed in the period of E₂d-E₂s.Activity of step-fault zones generally shows the cycle characteristics from weak towards strong to weak.Owing to different skew angles between advantageous extended orientation in different periods and terrace,Wubao terrace experienced the process of pure extension,dextral strike-slipping extension,pure extension,while Zhenwu terrace experienced sinistral strike-slipping extension,dextral strike-slipping extension,sinistral strike-slipping extension.The extension and strike-slipping of the south terrace happened mainly at the period of Funing,Danan,and Sanduo.The geometric characteristics,activity and genetic mechanism of Chenbao reverse fault are systematically analyzed.Chenbao reverse fault strikes to EW,incline to south in a high angle and mainly actives in the period of Funing to the peak at the period of Fusan.The formation of reverse fault is related with certain deep background and it belongs to gravitational sliding.Based on the experiment of sandbox,we stimulated the formation and evolution of the south terrace,and summarized the development modes of faults and basins in the south terrace.At the same time,we comprehensively analysis the tectonic relationships between basement fault and the south terrace.According to basement fault and the tensity of regional stress field,the newly-generated faults are classified into five genetic types which are direct succession,indirect succession,derivation,adjustment,and induction.Multistage rifting with different types is the key to diversified tectonic styles in the south terrace.According to cores and logging data,we comprehensively analysed the sedimentary facies of the south terrace,including fan delta,nearshore subaqueous fan,turbidite fan,braided river,lakes,and also the single well phases and cross-sectional phases of its key wells,the law of sedimentary evolution of the study area is made clearly.The result shows that the study area mainly develops fan delta due to its huge elevation difference and adequate material sources at K₂t,named basin forming stage.As a result of strong extension during E₁f stage,Gaoyou sag were settled down and transgressed by lake,which led the decrease of fan delta's scale.During E₂d stage,Gaoyou sag strongly developed as rift basin,with multiple sedimentary facies such as fan delta,nearshore subaqueous fan,turbidite fan.At early stage of E₁s,large area of delta facies was inherited from early time,and then shrinked to braided river facies at late stage of E₁s.This paper comprehensively analysed the relationships between the structure,the sedimentary features and hydrocarbon accumulation.Terrace structure,which had an obvious control on sedimentary,is systematically summarized and classified according to the amount of faults,the occurrence of faults,the strike of faults,tectonic activity and the features of paleogeomorphology.Distributary channel sandbody and turbidite sandbody are two types of favorable sandbody,which could further develop into two types of hydrocarbon accumulation:distributary channel sandbody mode in terrace and turbidite sandbody mode in depression.