Quantitative Logging Characterization Method Of Petrophysical Facies For Tight Sandstone Reservoirs In Kuqa Depression And Its Application

The lithology and lithofacies,diagenetic facies,fracture facies and pore structure facies of tight sandstone reservoirs in Lower Cretaceous Bashijiqike Formation in Keshen area of Kuqa depression are studied by making full use of core observations,normal thin sections,casting thin sections,cathode luminescence(CL)analyses,scanning electron microscope(SEM),mercury injection capillary pressure(MICP)analyses,conventional logging suits and image logs.The results show that the Bashijiqike Formation are formed in the sedimentary facies zone of braided delta front subfacies,of which the sedimentary micro-facies can be divided into underwater distributary channel,mouth bar and underwater distributary bay.The median grain size,sedimentary structure and compositional maturity index are chosen to further divide the sedimentary micro-facies into lithology and lithofacies,and seven types of lithology and lithofacies in Bashijiqike sandstones are divided: Medium-grained sandstone of underwater distributary channel,fine-grained sandstone of underwater distributary channel,coarse-grained sandstone of underwater distributary channel,sandy conglomerate of underwater distributary channel,medium-grained sandstone of mouth bar,fine-grained sandstone of mouth bar,and mudstones of underwater distributary bay.The logging characterization methods of lithology and lithofacies are built by observation and description of image logs and curve features of conventional log suits,combined with the logging calculations of medium grain size and compositional maturity index.The Bashijiqike reservoirs have experienced diagenesis such as compaction,cementation,and corrosion and fracturing,and it is now in the phase A of middle diagenetic stage.Then five types of diagenetic facies are divided based on the type and degree of diagenesis,diagenetic minerals and their effect on the reservoir quality.They are: tight compaction facies;carbonate cemented facies;mixed-layer illite-smectite filling facies;diagenetic micro-fracture facies;corrosion of unstable components facies.According to the various well logging response characteristics of different types of diagenetic facies,five conventional logging curves which are sensitive to the diagenetic facies are chosen to characterize the various diagenetic facies,and they are: neutron porosity logging(CNC),bulk density logging(DEN),sonic transit time(AC),natural gamma ray(GR),array induction log(M2R2),and thus the logging recognition model and standard of diagenetic facies could be set up combined with the mineralogical profiles obtained by Elemental Capture Spectroscopy(ECS)logs.Observation and description of core photos and image logs show that the fractures are well developed in the Bashijiqike sandstones.The fracture facies are divided into four types as: network fracture,high-angle fracture,low-angle fracture and horizontal fracture according to the fracture angle and fracture density.The fracture facies are recognized comprehensively by making full use of conventional log suits,image logs and array acoustic logging data.There are many kinds of pore spaces and pore throats with various radius developed in the Bashijiqike sandstones,and the types of combination between pore and throat are mainly mesopore leptothroat and fine pore-leptothroat.The reservoir pore structure can be divided into 4 types(Type ? medium pore radius and medium pore throat,Type ? medium pore radius and fine pore throat,Type ? small pore radius and fine pore throat,Type ? tiny pore radius and tiny pore throat)based on the mercury injection curves and the parameters obtained by the mercury injection experiments.The logging characterization methods of pore structure facies are built by optimizing and calculating the maximum pore radius which is sensitive to the pore structure by well logs.Then the petrophysical facies can be divided through the superimpositions and coupling of the lithology and lithofacies,diagenetic facies,fracture facies and pore structure facies,and then many types of petrophysical facies such as medium-grained sandstone of underwater distributary channel-corrosion of unstable components-network fracture-? medium pore radius and medium pore throat are divided in this way.Then the above mentioned four facies are compared with the actual property analysis data or gas testing data,therefore the quantitative characterization methods of each facies is formed by the method of weighting average in this way,and then the weighted value for lithology and lithofacies,diagenetic facies,fracture facies and pore structure facies is given considering the different contribution of the four facies to the property or gas productivity of Bashijiqike Formation tight gas sandstone reservoirs.Therefore the quantitative division and characterization standard of reservoir petrophysical facies can be built up in this way.Four main types of petrophysical facies(PF1,PF2,PF3,PF4)are summed out according to the constructive and destructive impacts of lithology and lithofacies,diagenetic facies,fracture facies and pore structure facies on the reservoir property of Bashijiqike reservoirs.The quantitative control of petrophysical facies on the reservoir validity is clarified according to the well logging interpretation and actual gas test results.Then the quantitative divisions of petrophysical facies for each well are realized by processing the actual logging data,and then the vertical distribution law of petrophysical facies in each single well is clarified.Then the horizontal distribution law of petrophysical facies is clarified by analyzing the connected wells profile.At last,the plane distribution of petrophysical facies of each single layer of Bashijiqike Formation are figured out,therefore the horizontal and plane distributions of favorable petrophysical facies can be used to predict the distbution of high quality reservoirs.