Fault Characteristics Of Chepaizi Uplift In Junggar Basin And Its Control To Oil-Gas Accumulation

The tectonic evolution of Chepaizi uplift in northwest Junggar Basin is a component of the tectonic evolution of Junggar Basin.Meanwhile,the evolution of this inverted triangle uplift has undergone a special process,which affects the migration,accumulation and distribution of oil gas.Multi-qualitative faults are developed in that area,including large reverse faults and numerous small normal faults.The multi-qualitative faults have controlled the formation and distribution of oil and gas reservoir in the study area,and the tectonic evolution of Chepaizi uplift has controlled the faults formation.The relationship between the formation mechanism of these faults and oil gas reservoir is one of the critical issues restricting oil gas exploration currently.Based on the analysis of geoligical data and geographic data,This paper studied the tectonic evolution of Chepaizi uplift.The choronology data of the regional tectonic deformation events,the sedimentary system of Chepaizi uplift,and the time sequence of tectonic deformation events of Chepaizi uplift are synthesized.The overall deformation time sequence of Chepaizi uplift in northwest Junggar Basin is re-clarified,the structural evolution process of Chepaizi uplift is recovered,and the formation mechanism of different types of faults controlled by the tectonic evolution process is discussed;Meanwhile,Fault one(F1)and Fault two(F2)which played an important role in controlling the oil gas accumulation in the study area are analysized.By combining the“qualitative analysis and quantitative analysis" method and making use of the research approach of“single factor classified evaluation and multi-factor fuzzy synthesis",the longitudinal and plane changing law of the closure property of F1 and F2 faults is elaborated,and its controlling effects on oil-gas reservoir are analyzed at last.As is indicated by the study results,wo sets of fracture systems will be developed on Chepaizi uplift,namely,the reverse fault which is formed in Hercynian-Indosinian period and cuts through the carboniferous system and the bottom of the cretaceous system as well as the normal fault which is formed in Himalayan period and cuts through the cretaceous system,the palaeogene system and the neogene system.Classify the faults of Chepaizi uplift into four different "levels":the faults of the first level are Hongche faults of NNE extension.They are the boundary faults of Chepaizi uplift and their fault displacements are large.The faults of the second level are those like F1,F2 which extend in the NNE direction to 5-10 kilometers and cut through the entire stratum.Their fault displacements are less than 25 meters and they are the most significant reservoir-controlling faults within the research area.The faults of the third level are reverse ones developed from the carboniferous system and ending at J-K systems,extending in the directions of NNE and NNW to 1-5 kilometers.And the faults of the fourth level are normal ones developed since N-Q.Their extension distance in EW direction is less than 1 kilometer and the positions of cutting through vary.Their fault displacements are 10 meters or so.The structural evolution process of Chepaizi uplift can be divided into four stages,namely,the intense uplift and initial development stage(C3-P),sustainable uplift stage(T-J),stable burial depth stage(K-E)and regional stretch and tilt stage(N-Q).In the four stages,there are differences in the tectonic stress background,which leads to the significant diversity of fracture system and the stratum thickness formed in different stages.In C3-P period,the Chepaizi uplift was located on the thrust fold belt moving eastward.The Hongche fault zone close to NS direction,second-level adverse fault and third-level adverse fault thrusting eastward are developed without stratum sedimentation;In T-J period,the right turn and compresso-shear activity of Chongche fault zone makes the Chepaizi uplift on the thrust wedge top belt,which is squeezed in north-east directions.Some of the adverse faults conducts inheritance-based sustainable development,some stops development,and others stops development with the formation of blind thrust fault after developing for a while.The stratum lapout occurs from the south and east to the north;In N-Q period,due to the strong uplift of Beitian Mountain which thrusts the Junggar Basin,huge structural load has been generated,making the southern rim deflected and submerged again.Chepaizi uplift is totally located in the foreland basin structural region of Beitian Mountain—Sikeshu sinking part.Beitian Mountain uplifts strongly and thrusts northward,and the Daerbute fracture starts to move to the left.Blocked by Zhayier Mountain,the stratum of this area conducts counterclockwise levorotation and twist for deformation,forming a series of level-four twisted normal fault in the north-west direction of the Cenozoic stratum.The second level faults(F1 and F2)start to invert negatively.As shown by the profile combination features,there are ladder,horst,graben,Y-shaped,flowerlike,inversion and other types distributed in the en echelon and pinniform on the plane.On the basis of structural research,this paper summarizes the oil gas accumulation mode of Chepaizi uplift by combining an elaborate analysis of the current oil gas accumulation.the hydrocarbon conduction accumulation model on the surface of fault-unconformity caused by four trees sag into Chepaizi uplift,the conduction accumulation model of the ladder type fault(caused by Changji sag through Hongche fault belt to Chepaizi uplift)-surface of unconformity and the fault(caused after oil and gas are transported to the bulging high place)-surface of unconformity-hypomere thick sand layer of Shawan formation form the complex conduction accumulation model of Chepaizi uplift.The two faults,F1 and F2,are of vital controlling importance to oil and gas accumulation of the bulging high place.With the use of the research method of "qualitative analysis combined with quantitative evaluation"and the research technique of“single factor classified evaluation and multi-factor fuzzy synthesis",the vertical and horizontal change laws of the closure of F1 and F2 are analyzed.The research results indicate that the overall change features change by sections on plane and longitudinal direction,and the vertical is divided into four stages of K,Nis1,Nis2 and Nis3.In the Cretaceous system stage(K),the fault sealing feature is overall good in flat surface,and only southern fault sealing of F2 fault is bad;In the Neocene sediment stage one(Nisl),F1 fault sealing feature has variation features with good northern,bad middle,better and worse southern,and F2 fault feature is overall bad-worse;for the Neocene sediment stage two(Nis2),fault sealing feature is overall good;In the Neocene sediment three-stage(Nis3).