Complex Reservoir Characteristics,Genetic Mechanism Of Lower Carboniferous Carbonate,Nanha Exploration Area

The study area of Nanha exploration area is located in the northwest of Chu-Sarysu gas-bearing Basin in Kazakhstan.The main Lower Carboniferous carbonate reservoir,the Upper Devonian clastic rock reservoir and Pre-Devonian metamorphic and granite reservoir were found during 80 exploration years from Soviet Union.The Lower Carboniferous reservoir has the characteristics of variety lithology,wide range physical properties,strong heterogeneity,and develops high production sweet.With the further exploration,reservoir description,genetic mechanism becomes a key problem.Based on core description,thin section observation,analysis and test,according to petrology,sedimentology,the characteristics and genetic mechanism of the lower Carboniferous complex carbonate reservoirs are studied.Limestone is the main lithology,dolomite,sand and evaporite are locally developed.12 lithofacies and 6 lithofacies association patterns,40 microfacies and 9microfacies association patterns are identified to classify sedimentary facies and study sedimentary model evolution.The platform has experienced from ramp sedimentary model to rimmed sedimentary.The reef-shoals complex distributes in the northwest and shoals distributes in the east and south.On the whole,the reef and shoals have the distribution pattern of the bank in early and the reef in late,the reef in the front and the bank in the back.Considering the high heterogeneity,reservoir space,physical property and porethroat structure are comprehensively characterized.Combination with production,three most important reservoirs re optimized,namely,residual structure pore dolomite reservoir,pore-vuggy reef limestone reservoir and fracture-pore grain limestone reservoir.Sedimentary face is the basis of reservoir development,the fresh water dissolution in the syngenetic stage formed abundant pores,reflux infiltration and dolomitization effectively inhibited compaction and pressure solution.Although cementation would reduce pores,the late fracturing can improve permeability and production both.Thick anhydrite on the top of Lower Carboniferous preserves the reservoir.Genetic mechanism of reservoir is controlled by sedimentary face,freshwater dissolution,reflux infiltration and dolomitization,fracturing and anhydrite cover.