Characterization And Comprehensive Evaluation Of ‘Sweet Spots’ In The Chang 8 Tight Fractured Sandstone Reservoir In Honghe Oilfield,Ordos Basin,China

The Honghe oil field is in the southwestern part of the Ordos Basin,China.The producing layers stratums is the Chang 8 layer of the Mesozoic Upper Triassic Yanchang Formation,which developed braided river delta front deposition,and the underwater distributary’s channel and estuary dam in the delta frontier are the mian microfacies.The production practice shows that the Chang 8 tight sandstones have complex pore-throat structure and strong heterogeneity.The oil-water relationship is complicated,and the difference between oil test and production is clear.the type and formation mechanism of Sweet Spots are unclear,the main controlling factors of Sweet Spots distribution are diverse,and the evaluation of tight sandstone reservoirs is difficult,which leads to the difficulty in optimizing Sweet Spots of tight fractured sandstone reservoirs.Therefore,finding out the formation mechanism and main controlling factors of Sweet Spots in tight fractured sandstones and carrying out the comprehensive classification and evaluation research of Sweet Spots in tight fractured sandstones have important demonstration significance for the efficient development and utilization of unconventional oil and gas resources in the southwest Ordos Basin and even the China.Based on studying plenty of literatures,this thesis makes comprehensive use of technologies and data such as outcrop-core observation,3D seismic data analysis,micro petrophysical experiment（constant pressure-rate mercury injection,cast thin sections,scanning electron microscope images,cathode luminescence images,nuclear magnetic resonance and micro CT scanning experiment）,conventional and imaging well logging interpretation,fluid analysis experiment,micro percolation experiment et al.Combined with the production test and production performance data,based on the research on the structure（structure and multi-scale fracture system）,stratum,sedimentation（sedimentary microfacies and reservoir heterogeneity）,diagenesis,micro pore throat structure,micro percolation characteristics and production performance of the Chang 8 member,the multiple main control factors of Sweet Spots reservoir of the target layer are discussed.Through the detailed analysis of various influencing factors,the comprehensive classification research scheme of tight fractured sandstone reservoir guided by the theory of ternary control of reservoir and the comprehensive classification evaluation scheme of tight fractured sandstone Sweet Spots guided by the five-dimensional control of accumulation/production mode are put forward.Based on the limit standard of production capacity under different oil prices,the optimization and prediction of Sweet Spots distribution of tight fractured sandstone in Chang 8 member of Honghe oilfield under different oil prices are completed.The main conclusions and understanding of this thesis are as follows:（1）The structure of the target layer is a monoclinal structure with northwest tendency（a slope of 6 m/km～8m/km）,and nose uplift is developed locally.The target layer mainly develops the braided river delta sedimentary system distributed in the southwest northeast direction.The framework sedimentary microfacies are the underwater distributary’s channel and estuary dam in the delta frontier microfacies.The vertical direction of multi-stage distributary channel shows shear superposition,vertical superposition,dislocation superposition and lenticular superposition.The plane continuity and connectivity of the sand body are good,the intralayer heterogeneity is strong,and the interlayer heterogeneity is weak.（2）The Chang 8 member mainly develops feldspar lithic sandstone.According to the main diagenesis,four types of diagenetic facies are divided:1-Chlorite grain coating and residual pores-dissolution facies;2-Clay mineral filling and residual pores-feldspar alteration facies;3-Clay mineral filling-weak dissolution facies;4-Ferro calcite/dolomite cemented-tightly compacted facies.Chlorite grain coating effectively improves the ability to resist compaction,combined with the development of dissolution pores,which constitutes the main pore type of Sweet Spots in Chang 8matrix reservoirs.Based on the response analysis of well logging to diagenetic facies characteristics,the single well logging identification and prediction of diagenetic facies is carried out based on the twice model optimization and Grid Search CV hyper-parameter determination method,which provides a basis for the study of plane distribution of diagenetic facies.（3）Two stages of hydrocarbon charging are mainly developed in Chang 8 member.The early-low maturity charging mainly occurred from the Late Jurassic to the late Early Cretaceous（140～110Ma）,which is the main charging period.The late charging mainly occurred in the Middle Cretaceous（110～79Ma）,which is the secondary period.The comprehensive relationship analysis of the main diagenesis stages,pore evolution history and hydrocarbon charging stages of the Chang 8 layer shows that the early hydrocarbon charging is first and then densification,and the early reservoir formation is dominated by lithologic reservoir.For the late charging,the target reservoir mainly manifested by first densification and then weak hydrocarbon charging.（4）The tight sandstone matrix reservoir of Chang 8 member is mainly composed of intergranular pores（mixed and primary intergranular pores）,and a small amount of intragranular dissolved pores are developed in the reservoir with high soluble components.Four types of throats are developed:constricted throat,systolic laryngeal throat,flaky throat,curved throat and tube bundle throat;5 types of pore-throat distribution curves are classified（7 pore-throat combinations）:intergranular-intragranular pore-constricted throat,intergranular-intragranular pore-flaky/curved throat,intergranular-intragranular pore-tube bundle throat,intergranular pore-flaky throat,intragranular pores-laky/curved throat,intragranular pores-tube bundle throat,intercrystal pore-tube bundle throat.Based on the curve characteristics of constant pressure-rate mercury injection experiment,a method for determining the division radius of connected pore-throat（intergranular pore）and non-connected pore-throat（intragranular and intercrystal pore）is established.By calculating the contribution of main storage space to reservoir physical properties（porosity and permeability）,three types of pore-throat structure models are divided:intergranular pore dominated-large pore and thin throat-ink bottle type,intragranular pore dominated-tree like model and mixed pore dominated structure model.Further,based on the fractal characteristics of wetting phase in total pore-throat space of tight sandstones,the above classification method of the division radius and matrix reservoir space is simplified,and the rapid classification method of matrix reservoir pore throat structure based on the total fractal dimension is realized.（5）The Chang 8 layer mainly developed large,medium,small and micro scales fractures.The large-scale strike slip fault system was mainly formed in the middle and Late Triassic,accompanied by four groups of small-scale NNE/NE-SE,EW,NNE/NE-SW and SN tectonic shear fractures.The small-scale fault fracture system is a five grade normal fault with complex strike and a transformation fault transformed by Himalayan movement.The micro scale fracture system is developed in each tectonic period and diagenesis-accumulation period.The large-middle scale strike slip fault zone has a large extension length and depth,and the downward charging and accumulation are formed by cutting through the upper source rocks.The fault-fractured trap developed based on large-middle scale fault system provides an efficient accumulation storage spaces for tight sandstone reservoir,and is an important control factor for the development of Sweet Spots of tight fractured sandstones.Middle-small scale fracture system mainly affects the development and distribution of Sweet Spots by improving reservoir physical properties（fracture porosity and fracture permeability）.The detailed evaluation of fracture system provides an evaluation basis for the effective hydrocarbon accumulation of fault fracture trap controlled by strike slip fault zone.Based on the analysis of fracture well logging response characteristics and single index amplification preprocessing,the fracture well logging identification and prediction model based on machine learning algorithm was established.It provides a basis for the evaluation of fracture distribution and the comprehensive classification and evaluation of tight sandstone reservoirs.（6）According to the experiments of constant pressure-rate mercury injection experiment,nuclear magnetic resonance and micron CT experiment,the main controlling factors of tight sandstone reservoir permeability in Chang 8 member are determined:the main throat radius and R₂₀.The amount of large pore-throat greater than 1μm among various pore-throat structural parameters calculated from constant rate mercury injection experiment showed good correlation with K.R_NM50among pore-throat structural parameters calculated by T₂curves showed best correlation with K.The average pore throat ratio calculated from Micro CT experiment showed good correlation with K.It is showed that large intergranular pore throat ratio are the best evaluation parameters for tight sandstone reservoir permeability.The saturation/centrifugal NMR experiment results show that the main controlling factors of movable fluid content in tight fractured sandstone reservoir are pore-throat structure parameters and fracture intensity.Pore throat ratio is the main controlling factor of the efficiency of mercury withdrawal.（7）Based on the determination of the lower limit of the reservoir,the classification of tight sandstone matrix reservoir of Chang 8 member（three types of matrix reservoir and one non reservoir）is established by integrating the analysis of sedimentary,digenesis,storage performance,pore-throat structure,seepage parameters and movable fluid distribution parameters.The best type of matrix reservoir is dominated by intergranular pore type.Combined with the research on tectonic reservoir control,the comprehensive classification and evaluation standard of reservoir guided by the ternary reservoir control theory is established,and the comprehensive classification research of tight fractured sandstone reservoir in Chang 8 member is completed.The fracture type,pore-fracture type and pore type reservoirs are divided,and high-quality reservoirs are concentrated in the fracture type and pore-fracture type reservoirs.（8）Based on the previous study,through the detailed analysis of the main controlling factors of the Sweet Spots in tight fractured sandstones,the theory of“ternary controlled reservoir（structure（multi-scale fracture system）,sedimentation（facies controlled sand body and macro heterogeneity of reservoir）,diagenesis（micro nano pore throat structure,movable fluid and mobility,micro oil displacement mechanism））”and the theory of“five element control of accumulation and production（structure,sedimentation,diagenesis,hydrocarbon charging（source rock thickness,source reservoir configuration and fracture system）,productivity/producibility（fracture,microfacies,diagenetic facies,micro pore throat,movable fluid,source rock and micro oil displacement mechanism））”are improved,which provides a reference for the efficient development of unconventional tight fractured sandstone reservoirs.（9）Based on the comprehensive evaluation,through the definition and classification of Sweet Spots,the distribution and production characteristics of different types of Sweet Spots are discussed.On this basis,using the Grey Correlation Clustering Technology and guided by the theory of three element control of storage and five element control of accumulation and production,five kinds of parameters are optimized,the comprehensive evaluation model of Sweet Spots Oil Layer is established,and the distribution principle of Sweet Spots Oil Layer under different oil prices is predicted.