Structural Characteristics Of The Faults And Their Controlling Law On Hydrocarbon Distribution In Southern Jinhu Depression

The Jinhu Depressioin is located in the southwest of Subei Basin.The fault system in the Jinhu Depression is extremely complex,and there are thousands of faults in different directions,horizons,properties and levels,which are controlled by the basement pre-existing structures of the Indosinian and Yanshanian periods and regional tectonic stress fields in different periods.However,the genetic mechanism of the fault system and the controlling law on hydrocarbon distribution in Jinhu Depression have not yet been deeply understood.Therefore,it is of great theoretical and practical significance to carry out the structural analysis of the fault system and its control on hydrocarbon accumulation in Jinhu Depression.In this paper,the distribution,activity and evolution characteristics of the complex fault system in the south of Jinhu Depression were systematically analyzed by using the data of regional geology,seismic,drilling,and combining a series of techniques and methods,including the three-dimensional seismic interpretation,quantitative analysis of fault activity,balanced cross section,structural numerical simulation and structural physical simulation,and the influence of Yangcun fault growth and linkage on the evolution of Jinhu sag was discussed.Based on the analysis of regional tectonic dynamics and stress field,the local stress field at different stages of fault growth and linkage,the local stress deflection effect of the pre-existing fault in the basement,the relationship between the thickness of the cap layer,the disturbance degree of the pre-existing fault,the progressive deformation of reactivation of the pre-existing faults were numerically simulated,and then the genetic mechanism of the fault system was proposed.Finally,the controlling effects of faults on different accumulation factors were discussed respectively,and the fault controlling laws on hydrocarbon distribution were summarized.The results show that according to the differences of fault distribution,the southern part of Jinhu Depressioin could be divided into three regions:Western Region（I）,central region（II）and eastern region（III）,and then subdivided into six sub regions,including Nipei-Wanglongzhuang sub region（I-（1））,Chajian-Qinying sub region（I-（2））,Tongcheng West sub region（II-（1））,Tongcheng East sub region（II-（2））,Bianminyang West sub region（III-（1））and Bianminyang East sub region（Ⅲ-（2））.There are significant differences in fault structural styles in different zones,but they reflect that the tectonic background dominated by extension,supplemented by strike-slip.The activity of Yangcun fault was the strongest in the deposition period of E₁f₄.Spatially,the activity intensity of Yangcun fault was the highest in the Longgang sub-depression and gradually weakened to both ends.The difference of fault activity resulted in the weakening of control effect on depression from west to East,the“half graben”developed in the central and western parts,and“nearly symmetrical graben”developed in the eastern parts.According to the theory of fault growth and linkage,Yangcun fault can be divided into four segments:Qinying segment（QYS）,Chajian segment（CJS）,Longgang segment（LGS）and Minqiao segment（MQS）.Minqiao segment can be divided into eastern Minqiao segment（MQES）and Western Minqiao segment（MQWS）.From the Late Cretaceous to Paleogene,the growth and linkage of Yangcun fault could be divided into four stages:isolated segment growth stage（K₂t-E₁f₁）,fault soft linkage interaction stage(E₁f_2-3),fault hard linkage through stage（E₁f₄）,and late finalization and transformation stage（E₂d-E₂s）.During Funing period（E₁f）,the Yangcun fault changed from isolated segmented sub-depression control to overall through depression control.Controlled by the growth and linkage of Yangcun fault,Jinhu Depression converted from scattered fault depression to concentrated fault depression.The numerical simulation of fault shows that the“tip effect”of the minimum principal stress at the end of fault was the mechanical mechanism of fault transverse propagation.When the tip of overlapped fault entered the stress drop area,the fault transverse propagation was blocked,the accumulation of displacement caused damage of relay ramp and segmented fault linkage.Compared with the oblique extension,the superimposed tensile stress field under the Orthogonal extension was stronger,and it was easier to break the relay ramp and caused the fault linkage.The local stress field disturbed by the pre-existing faults in the basement,resulting in the strike deflection of the secondary faults near the faults,and the deflection degree was mainly controlled by the distance from the basement faults and the angle between the extension direction and the strike of basement pre-existing fault.Where the thickness of the cap rock was thin,and the basement pre-existing fault disturbed the stress field of the cap rock strongly,so it was easy to reactivate in the later extension.In the early stage of progressive deformation,the pre-existing faults reactivated and propagated along the direction nearly perpendicular to extension;in the late stage of progressive deformation,the shear strain zone controlled by pre-existing faults gradually transformed into those controlled by regional extension.Based on the comprehensive analysis of fault geometry,kinematics and dynamics,the genetic evolution model of the Yangcun fault and the Tongcheng fault in the syn-rifting period were proposed respectively.The Yangcun fault was mainly controlled by the growth and linkage of the reactivated basement faults,and its genetic evolution model could be divided into four main evolutionary stages:K₂t-E₁f₁,E₁f_2-3,E₁f₄-E₂d and E₂s.The Tongcheng fault was mainly controlled by the differential rise and fall between the CJS and LGS,and its evolution stage could be divided into three stages:the initial differential rise and fall embryonic stage(E₁f_2-3),the stable differential rise and fall propagative stage（E₁f₄-E₂d）,and the strike-slip superimposed differential rise and fall finalization stage（E₂s）.The genetic mechanism of complex fault system in south of Jinhu Depression mainly included two stages of extension direction change,fault growth and linkage,basement pre-existing tectonic stress disturbance and pre-existing fault reactivation-progressive deformation.The extensional direction depends on the relative strength of regional tectonic dynamic sources.During the K₂t-E₂d,the extension of the NNW-SSE dominated by the subduction of the Pacific plate;during the E₂s,the near SN extension dominated by dextral strike-slip of the Tanlu fault.Based on the relative strength of regional extension and basement fault control on fault growth and linkage,fault growth and linkage could be divided into two types:regional extension control type and basement fault control type.The strike deflection of the secondary faults in the downthrow of Yangcun fault were caused by the tectonic stress disturbance of the pre-existing faults in the basement,among which the NE strike segment had a large deflection amplitude,while the NEE strike segment had a small deflection amplitude.The reactivation and progressive deformation of pre-existing faults resulted in the reactivation of NE,NEE and NW pre-existing faults and their propagation along nearly vertical extension direction,forming a series of arc-shaped faults.The tectonic evolution of the complex fault system in the south of Jinhu Depression was generally controlled by the NE and NEE strike basement faults in the Yanshanian and Indosinian periods,the twice changes of extension directions in the E₂s.Based on the different controlling effects of the fault system on the Jinhu Depression in different evolutionary periods,the tectonic evolution of the fault system in synrift phases in the southern Jinhu Depression could be divided into three stages:synrift discrete fault depression stage（K₂t-E₁f₃）,synrift concentrated fault depression stage（E₁f₄-E₂d）and synrift strike-slip superposition stage（E₂s）.In terms of time,the changes of activity intensity of the Yangcun fault in different periods controlled the thermal evolution stages of source rocks in the rifting stage;in terms of space,the activity intensity and mode of the Yangcun fault controlled the distribution of mature source rocks of E₁f in the southern Jinhu Depression.The NEE and NE strike basement faults and the NNW-SSE trending tensile stress field during K₂t-E₂d jointly controlled the formation of the main faults controlling the traps and fault block and fault nose traps in the south of Jinhu Depression.The fault-caprock configurations controlled the hydrocarbon accumulation horizons in the south of Jinhu Depression.The differences of extension mode and activity for the different segments of Yangcun fault was the main reason for the difference of hydrocarbon distribution in the south of Jinhu Depression.