The Tectonic Characteristics And Oil & Gas Distribution Of Shaobo Subsag And Its Surrounding

Located in Gaoyou deep sag,Shaobo Subsag,Fanchuan Subsag and Liuwushe Subsag are the rich oil and gas accumulated belts in Gaoyou depression.Due to its complex structural features,there are still some debates on its genetic evolution and mechanic process.Therefore,it is quite important to make the deep analysis on this topic and make clear its control on oil and gas accumulation for further exploration.Besides that,the Shaobo Subsag is a typical strike-slip and extensional subsag.To understand its formation mechanics is important to help us know the similar complex basin and the oil and gas exploration in them.Based on study of the fine seismic interpretation,well logging data and fine analysis on3D seismic attribution analysis,the geometry,kinematics and dynamics of all structures are studied,and the genetic model of the deep sag belt and Shaobo Subsag was constructed.Through the study of oil and gas accumulation mechanism,trap type and distribution,reservoir characteristics,the relationship between reservoir and traps,the favorable oil and gas exploration zone,favorable area,favorable trap and excellent oil and gas exploration zone in Shaobo Subsag and its surrounding area were determined and the exploration targets are selected.According to the duration of fault activity,fault size and their control on basin and internal structures and influence on sedimentation,the faults around Shaobo Subsag can be divided into five ranks.The main fault is Zhen（1）fault,which divides Gaoyou depression and Tongyang uplift,and is the southern boundary fault of Gaoyou depression;the secondary fault is Zhen（2）fault and Hanliu fault,which constitute the southern and northern boundary of deep depression.The fault profile in the study area is mainly plate type and listric type while the ramp-flat faults mainly develops in the Zhen（1）fault.Planar faults are mainly characterized by multi-segment arcs,and linear faults are the most developed secondary faults.The structural style of the study area includes not only extensional structural style,such as step faults association,synthetic faults association,antithetic faults association,graben and Y-shaped faults association,but also strike-slip construcural style which were presented the flower-like and brush-like combination styles in the section,en echelon,oblique and feather like combination styles on the plane.According to the geological dynamic background,structural evolution,stress field simulation,mechanical properties and formation mechanism,the evolution of Shaobo Subsag and its surrounding area can be divided into four main periods:Taizhou to the second member of Funing Formation,the third and fourth member of Funing Formation,the Daian period and the Sanduo period.After the deposition period of Yancheng formation,faults in this area are weaken and disappeared.Zhen（1）fault has a long time of movement,from late Cretaceous to Neocene.The movement of main faults started from east and middle area,then extended to the West.Fault Movement intensity in the East is relatively high,with the highest in Zhenwu area.The starting time of the Zhen（2）fault is E₁f₄.The faulting intensity and velocity of Zhen（2）-1,Zhen（2）-2 and Zhen（2）-3 are different,and Zhen（2）-1 and Zhen（2）-2 have higher velocity.During E₂d₂,Zhen（2）-2 had the highest velocity.The Hanliu fault began to develop in the Daian formation period with earliest in western part.The evolution of regional tectonic stress field in Subei basin can be divided into three stages.Before the early Cretaceous,the stress field was dominated by NW or sinistral shear compression.After the late Cretaceous,with the joint action of the Pacific plate and the Indian Ocean plate,the Subei basin experienced the alternating sinistral and dextral shear forces.Since Eocene,the acceleration of the subduction rate of the Pacific plate produced a back-arc tension,and Tan Lu fault zone had the dextral strike slip,which changed the regional stress of Gaoyou Depression into tension in north-south direction.The results of the stress field simulation indicate that the stress field in the study area is mainly characterized by dextral shear and NS direction tension.Regarding to genetic model problems in Gaoyou Depression,a new research method is proposed,including the study of structural geometry,kinematics,regional stress field,the study of the controlling effect on depression of boundary fault and its combination,internal fault and its combination with boundary fault,the construction of deep concave belt and its sub concave genetic model.By this new method and modeling,it is realized that the Gaoyou deep depression belongs to pull apart basin.The NWW section of Zhen（2）-1 and Zhen（2）-2faults divide and separate the deep depression into Shaobo Subsag,Fanchuan Subsag and Liuwushe Subsag.The intensity of strike-slip extension controls the enrichment of oil and gas in Subei basin,Gaoyou sag and Shaobo Subsag.Compared with the two major depressions in Subei basin,the intensity of strike slip extension in Dongtai depression is much stronger than that in Yanfu depression.Strike slip and extensional structures have four effects on oil and gas enrichment:1)control the distribution and quality of source rock,2)control the quality reservoir and cap rock and their combination,3)control the trap types and their distribution,and 4)control the differential migration and accumulation of oil and gas.By our study,there are three types of reservoir modes existing in Shaobo Subsag and its surrounding area,including faulted reservoir,lithologic reservoir and composite reservoir.Based on the analysis of oil and gas accumulation mechanism,trap type and distribution,reservoir prediction,three favorable areas for oil and gas exploration and four drilling wells are proposed.