| Chepaizi area is located in the northwest of the Junggar basin,and the Carboniferous is the key strata in the exploration and development of the region.The reservoir type is mainly the volcanic-fractured reservoir.The study of volcanic fracture in this area is not deep,and its development and formation mechanism,main control factors and its oil control are not clear,the analysis of the genetic mechanism of volcanic fractures in this area and the study of fracture distribution characteristics under different controlling factors are carried out in order to clarify how fractures control the law and model of oil and gas enrichment.This paper is guided by the relevant theories of reservoir geology and structural geology,using core,rock slice,earthquake,logging,oil test and production test data,etc,especially a variety of experimental analysis data of rock mechanics,the types and characteristics of fractures,the characteristics of tectonic dynamics,the mechanical characteristics of volcanic rocks and the law of fracture controlled oil and gas enrichment are studied systematically.The following main achievements and understandings have been obtained.It clarifies the effect of the structure of different tectonic movements on the fracture and the characteristics of fracture development.The study area is divided into four main tectonic stages: the middle Hercynian movement,the late Hercynian movement,the Indo-Yanshan movement and the Himalayan movement.Study on the influence of tectonic movement on the trend,scale and filling degree of fracture development in different stages.Among them,in the middle period of Hercynian,the near EW high angle transverse fracture perpendicular to the main axis of the structure is produced,but the fracture formation time is earlier.After strong tectonic evolution and diagenetic process in the later stage,most of the fractures are filled with calcite,and the effectiveness of the fracture is poor.The late stage of Hercynian is the main forming period of the structural fracture in the study area,where the X-type common-shear fracture is the main type of the fracture,and the fracture is more developed in the whole,but the filling degree is high.The tectonic stress field in the Indosinian-Yanshanian period is changed from the compressive stress to the pressure-torsion-slip stress,especially under the action of the slip-slip stress field,and the fracture width is expanded by the tension,so as to produce a series of expansion joints,which has better transformation effect on the fracture formed in the earlier stage.Indochina-Yanshanian tectonic stress field changes from compressive stress to compressive torsional strike slip stress,especially under the action of the slip-slip stress field,and the fracture width is expanded by the tension,so as to produce a series of expansion joints,which has better transformation effect on the fracture formed in the earlier stage.The fractures formed during the Himalayan period are mainly tensile and shear fractures parallel to the normal fault,but the fractures are small in scale and small in number,mainly based on unfilled or semi-filled high-angle shear joints,with high effectiveness.Revealed the rupture mechanism of volcanic rock and clarified the genetic characteristics of the fracture.According to the experimental results of rock acoustic velocity,rock fracture,hardness and triaxial compression of rock mechanics,it is considered that the Youngundefineds modulus,shear modulus and compressive strength of volcanic rocks with different lithology are different.The basalt and tuff in the area are more resistant to elastic deformation,and the rock is not prone to rupture.The basaltic breccia and basaltic breccia lava have poor resistance to deformation,and the rock is weak in resistance to external forces and is prone to rupture.The rock fracture development degree of the same lithology is also affected by the depth of the burial.With the increase of the depth,the surrounding pressure of the rock is increased,and the elastic modulus of the rock and other parameters are correspondingly increased,the compressive strength of the rock is increased,and the formation difficulty of the crack is increased.On the plane,the degree of rock fracture in the study area is divided into grade I fracture area,grade II fracture area and grade III fracture area.The control effect of unconformity structure on the cause of fracture is found out.In the range of 400 m from the unconformity surface of volcanic rocks in the study area,all the fractures are developed.The fractures is relatively developed in the range of 0-80 m from the unconformity surface,but the filling degree is high.In the range of 80-200 m from the unconformity surface,the fractures are more developed,the filling degree becomes worse,and there are more semi-filled joints or unfilled joints,which is the most favorable oil and gas reservoir section in the study area.In the range of 200-400 m from the unconformity surface,with the increase of buried depth,the degree of fracture development becomes weaker.The genetic types and main control factors of volcanic fractures are clarified.It is considered that the main factors affecting fracture development in the study area are tectonic stress and fault,litholithofacies and unconformity,among which tectonic stress and fault,litholithofacies are the main controlling factors.The low-angle shear fracture that is consistent with the origin of the reverse fault in the Carboniferous system reservoir in the study area is the main cause of the fracture.With the increase of the distance from the fault,the fault grade decreases,and the width and density of the fault symbiotic fracture decrease.From the point of view of litholithofacies,volcanic fractures are generally developed,but the degree of fracture development is different in different volcanic facies.In the overflow phase,andesite and basalt fractures are most developed,followed by volcanic eruptive tuff,and less sedimentary tuff fractures in volcanic sedimentary facies.The distribution characteristics of volcanic fractures are expounded.Based on the study of fracture distribution characteristics of fault,lithologic lithofacies and unconformity surface,combined with the response relationship between prestack seismic data and fractures,the fracture plane distribution in the study area is divided into four regions.Two of them are the most favorable,mainly distributed in the eastern and south-central regions of the study area.The model of the control of oil and gas accumulation in the carboniferous system fracture body is established,and the role of the volcanic crack control oil is defined.It is considered that the main reservoir period of the Carboniferous system reservoir in the study area is the Late Cretaceous in the late Yanshan period.During this period,the transportation and occlusion of fracture zone coexist,and it is easy to form oil and gas accumulation.Nowadays,the buried hill trap of bedrock in the Carboniferous system accumulates and forms reservoirs.and the fracture body determines whether the oil and gas are enriched or not.The development and filling of the fracture is the key to the reservoir performance of the fracture body.In view of the problems existing in the study of the genetic characteristics of volcanic fracture in non-coring well,a more effective solution is put forward,which provides technical support for the study of volcanic fracture.The achievements and understandings have been applied to further exploration in the study area in time and have produced good results. |
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