Fine Logging Description Of Sedimentary Reservoir Of Cretaceous In Bozi-Dabei Area Of Kuqa Depression

The proven natural gas geological reserves of Cretaceous Bashijiqike Formation（K₁bs）and Baxigai Formation（K₁bx）in Bozi-Dabei area of Kelasu structural belt in Kuqa depression are over trillion cubic meters.Reservoir quality evaluation and prediction have become the key to oil and gas exploration and development.The sedimentary microfacies of Cretaceous reservoirs in Kuqa Depression are relatively favorable for the formation and development of reservoirs.Therefore,it is necessary to conduct in-depth research on Cretaceous sedimentary facies in the study area,establish logging identification methods for different sedimentary microfacies,and clarify the control effect of sedimentary microfacies on the development of high-quality reservoirs.In this thesis,comprehensive geological and logging data,using conventional logging,image log,core,particle size probability cumulative curve and geological model method,established a typical sedimentary microfacies logging identification chart datebase.There are three sedimentary systems and five sedimentary microfacies in Cretaceous in Bozi-Dabei area,namely,braided river delta sedimentary system developed in the second member of Bashijiqike Formation（K₁bs²）,fan delta sedimentary system developed in the third member of Bashijiqike Formation（K₁bs³）,and lake delta sedimentary system developed in the Baxigai Formation（K₁bx）.The main sedimentary microfacies include underwater distributary channel microfacies,underwater distributary bay microfacies,distributary mouth bar microfacies,sheet sand microfacies and shore-shallow lake microfacies.The paleocurrent direction of Cretaceous in the study area was restored by imaging logging.The results show that the Cretaceous in the study area presents a dual provenance supply pattern dominated by the south Tianshan provenance supply direction from north to south,supplemented by the Wensu bulge in the southwest.The corresponding relationship between sedimentary microfacies and reservoir physical properties,diagenesis and fracture development shows that the physical properties of underwater distributary channel microfacies and distributary mouth bar microfacies are the best,followed by sheet sand microfacies,the shore-shallow lake microfacies and underwater distributary bay microfacies are the worst.The underwater distributary channel microfacies and distributary mouth bar microfacies have strong hydrodynamic force,the formed sand bodies are relatively pure,with coarse particle size,good sorting and strong anti-compaction ability.Therefore,it is easier to preserve the primary intergranular pores in the burial process.At the same time,corrosion is easy to occur in the later stage,forming dissolution pores.However,the bottom of underwater distributary channel sand body is often adjacent to the microfacies between underwater distributary bay,which is prone to strong carbonate cementation,resulting in poor reservoir physical properties.This is mainly due to the fact that Ca2+and Mg2+in mudstone deposited by the microfacies between underwater distributary bay are easy to migrate to the adjacent sand body in the diagenetic evolution process,thus carbonate cementation occurs.Underwater distributary channel microfacies sand bodies with thin layer（thickness≈3meters）often develop reticular fractures,which have the best improvement effect on reservoir physical properties.High angle fractures are often developed at the top and bottom of thick layer sand bodies（thickness>10 meters）,and low angle fractures are often developed in the middle.The coupling relationship among sedimentary microfacies,diagenesis and fractures with favorable reservoir development shows that sand bodies in high-energy hydrodynamic environment have experienced diagenesis of medium compaction and weak cementation,fractures are superimposed to form favorable reservoirs.Comprehensive evaluation of Cretaceous tight reservoir in Kuqa Depression and guidance for favorable reservoir prediction can be achieved by studying sedimentary microfacies and evaluating diagenetic intensity and fracture development characteristics under sedimentary microfacies constraints The research results can provide theoretical guidance and technical support for the fine description of Cretaceous sedimentary reservoirs and the design of development schemes.