Fracture Characterization And Effectiveness Evaluation Of The Bashijiqike Formation Tight Sandstones,in The Kuqa Depression

The Kelasu Belt,located in the northern part of the Kuqa Depression,Tarim Basin,contains a number of thrust fault-related anticlines produced in response to the Cenozoic tectonism.The Lower Cretaceous Bashijiqike Formation of this belt,a principal hydrocarbon target,has accumulated in a fan or braided-river delta environment,which is buried at depth more than 6000 m.The tight nature of this unit likely reflects the effects of diagenesis and compressive dynamics.The matrix porosity ranges between 1.5% to 5.5% and the matrix permeability is generally less than 0.1 mD.Natural fractures of tectonic and atectonic origins are extremely abundant in the Bashijiqike Formation and may serve as the main channel of fluid flow.Hence,the characterization of natural fracture is significantly important for the prediction of preferable migration channels,hydrocarbon enrichment areas and productivity evaluation,and also for fracture modeling.In this study,seismic data,borehole imaging logs,cores,casting sections,cathode luminescence(CL),scanning electron microscope(SEM)and productive data are integrated.The study respectively refers to fracture identification,fracture formation and evolution,the controlling factor analysis of fracture density,fracture pattern and its effectiveness analysis.Some conclusions can be drawn below:(1)Natural fracture and induced fracture are both developed in the Bashijiqike Formation.Furthermore,natural fractures can be classified into tectonic fractures,natural hydraulic fractures(NHF)and diagenetic fractures.Diagenetic fractures can be subdivided into intragranular microfractures,grain boundary microfractures and diagenetic shrinkage microfractures.Based on the tectonic and burial history,diagenetic and paleogeothermal evolution and hydrocarbon charging history,a probable fracturing sequence could be speculated as follows.The Bashijiqike Formation experienced its main fracturing period during the Niogene and Quaternary time.Early fracturing during the Middle Himalayan period(23.7 Ma~5.5 Ma)was simultaneously cemented.Meanwhile,resulted from the rapid subsidence,intragranular microfractures were widely distributed which was filled by calcite and authigenic quartz.Moreover,fractures(e.g.tectonic and natural hydraulic fractures,grain-boundary fractures and diagenetic shrinkage fractures),formed after the Late Himlayan movement(after 5.5 Ma),show a good hydraulic conductivity due to the low degree of cementation.Intragranular microfractures,developed during this period,were induced by the tectonic compression.(2)Fracture intensity in the Bashijiqike Formation is affected by lithology,sedimentary microfacies,diagenetic facies,mechanical stratigraphy thickness,fault and tectonic stress.Generally,the preferable geological settings for fracture development include fine sandstone,underwater channels,strongly compacted diagenetic facies,thinner mechanical layers,distance to faults less than 1km and high paleo-tectonic stress.Furthermore,the calculation of variable coefficient of fracture density indicates that mechanical stratigraphy thickness and tectonic stress are the key controlling factors which significantly affect fracture intensity.Fracture intensity gradually decreases from the Southern Tianshan Orogen to the depression.(3)Based on fracture attitude correction,fold-related fracture patterns in boreholes and core-image integration,fold-related fracture patterns in the Bashijiqike Formation include pre-folding shear fractures at the top of bedding plane,tensile fracture,bedding-parallel fracture and two pairs(four sets)of hinge-parallel,perpendicular or oblique conjugate fractures in the vertical section.Thereinto,two pairs of conjugate fractures in the vertical section are the majoring fracture type and their strike is closely related to the bedding strike.The image interpretation indicates that only one or two sets of the conjugate fractures are generally well developed in the single petroleum well.Fault-related fractures are poorly developed and appear as the configuration of two sets of conjugate fractures and one set of tensile fracture.(4)Diagenetic cementation overall decreases fracture effectiveness in the Bashijiqike Formation,thus resulting in the majority of fractures filled by mineral particles except part of conjugate fractures in the vertical section.The total fracture permeability is attributed to the four sets of conjugate fractures in the vertical section and permeability of the single set fracture is mainly controlled by fracture aperture and fracture spacing.Fracture aperture derived from electrical imaging was corrected by overburden pressure test,thus for the determination of fracture permeability.The results indicate that permeability of the single set of fracture changes between 0.19 mD and 55.73 mD.Furthermore,the total fracture permeability shows a good linear relationship with petroleum productivity.Fracture effectiveness is controlled by fracture sets,fracture aperture and fracture spacing.Based on the productivity test,the scheme of fracture effectiveness in the Bashijiqike Formation was proposed.