The Influence Of Structural Diagenesis On Natural Fracture Effectiveness Of Tight Sandstone Reservoirs

The dense reservoir is an important type in the continental sedimentary basin in China.Compared with conventional reservoirs,the matrix properties of the dense reservoir are poor,its heterogeneity is stronger and the natural fractures are more widely developed.Jointly influenced by tectonic movement and diagenesis,the distribution law and the effectiveness of fractures in the dense reservoir are complex and difficult to evaluate.Therefore,it is an important way to study the effectiveness of dense reservoir fractures by starting from tectonic-diagenesis.This research takes Jiange structures in the northern segment,Xinchang structures in the middle segment,and the second segment of Xujiahe Formation in the Upper Triassic of Baimamiao-Qiongxi area in the southern segment of the foreland depression belt in Western Sichuan Province as subjects.Based on the materials and analysis of the field outcrops,core samples,thin sections and logging data,this research studies the differences in the tectonism,diagenesis,reservoir properties,natural fractures and their effectiveness among different structure segments.Meanwhile,the difference controlling mechanism of tectonic and diagenesis interaction on the effectiveness of dense reservoir fractures and reservoir quality is clarified,and the control mode of the effectiveness of dense reservoir fractures based on structural diagenesis is established.The strength of different tectonic segments of the foreland depression belt in Western Sichuan is in alternating extrusion deformation.In the late Indosinian and Yanshanian,the structural deformation is strong in the northern and middle segment,and weak in the southern segment;in the Himalayan,structural deformation is strong in the south and weak in the north and middle.Affected by the deformation of the Mesozoic structure,the structural settlement in the northern and middle segments is greater than in the southern segment.The differences in structural deformation and settlement control the diversities in diagenesis,which affects the rock mechanical properties,structural deformation mode and fracture strength.The earlier the deformation happened,the stronger the deformation becomes and the faster the settlement is.As a result,the diagenesis is more obvious and the reservoir is easier to be densified,thus the properties are poorer.With the continuous progress of diagenesis,the mechanical properties of the rock deformation mode change from plastic deformation,deformation band to failure by rupture.Under the intensive diagenesis compaction,diagenetic fractures like bedding fractures,intragranular fractures and grain boundary fractures are formed.At the same time,different diagenetic facies are embodied with different rock mechanical properties,which control the development degree and distribution law of structural fractures.In the process of reservoir evolution,tectonism and diagenesis interact and influence each other.Due to the different structural deformation intensity,structural subsidence,structural uplift and burial depth of the target layer in different structural segments,the diagenesis of different structural segments is different.Tectonism controls the type and intensity of diagenesis,diagenetic evolution sequence,diagenetic evolution stage,development degree of diagenetic system and differences of diagenetic facies.The early diagenesis determines the degree of reservoir densification.With the continuous improvement of the degree of densification,it is conducive to the formation of structural fractures and significantly improves the physical properties of the tight reservoir,especially the seepage capacity of the reservoir;After the reservoir is fully densified,diagenesis determines the filling characteristics and effectiveness of natural fractures.In the different structural segments of the Western Sichuan Basin,the distribution and effectiveness of natural fractures in the tight reservoirs significantly varies.The development degree of fractures is controlled by the intensity of tectonic deformation,diagenetic facies,lithology,layer thickness,sedimentary microfacies,and abnormal high pressure.The fracture effectiveness is combined affected by the fracture development,tectonism and diagenesis.According to the analysis and experiment of mineral bridges,the evolution pattern of the crack-seal texture in different minerals bridge types and different fracture aperture.Furthermore,based on the calculation results of emergent threshold and index of structural diagenesis,the controlling of the structural diagenesis on the effectiveness of fractures and quality of reservoirs were elucidated.The intensity of the early structural diagenesis is the vital geological factor causing the compactness of the reservoirs.Simultaneously,the late structural diagenesis influences the effectiveness of fractures in tight reservoirs,and controls the distribution of the high-quality reservoirs.With the increase of the tectonism intensity,fractures are more developed and the fracture effectiveness is better.Therefore,the relation between the structural and diagenesis in late stage is the key factor that determine the effectiveness of fractures in tight reservoirs,as well as the improvements in reservoir physical properties.