Sedimentary Microfacies And Reservoir Characteristics Of The Chang 8 Formation In S118-SZ1318 Wellblock

S118-SZ1318 Wellblock,located in the south of Ordos Basin,is a relatively low degree of exploration block.In recent years,good oil and gas shows have been seen in the Yanchang Formation in this area,but the current geological understanding has been difficult to meet the actual needs of exploration and development in this area.In this paper,the sedimentary and reservoir characteristics of Chang 8 reservoir group in the study area are studied in depth in order to provide a reliable geological basis for further development.According to the marker layers,the target layers in the study area are carefully divided and compared,and the stratigraphic skeleton profile network of Chang 8 reservoir group in the study area is established.On the basis of systematic analysis of sedimentary facies markers such as petrology,grain size characteristics,sedimentary structure and logging curves,the types of sedimentary microfacies,microfacies assemblages and distribution characteristics of sand bodies in each sand formation are studied.Reservoir characteristics and controlling factors are mainly studied,and reservoir evaluation and classification are carried out.The main ideas are as follows:The Chang 8 reservoir group in the study area belongs to delta-lake sedimentary system.Microfacies assemblage includes three types: superimposed underwater distributary channel,interacting underwater distributary channel and Bashang river.The main types of diagenesis are compaction,cementation,metasomatism and dissolution,and the sandstone of Chang 8 reservoir in the study area is in the A-B stage of middle diagenesis.Sandstone is mainly fine-very fine-grained feldspar sandstone,and the pore type is mainly dissolution intergranular pore,which belongs to low-porosity-ultra-low permeability reservoir and is small-pore-micro roar pore structure.Generally speaking,Chang 8 reservoir in the study area is mainly controlled by sedimentary,diagenetic and pore structure factors.According to various parameters,Chang 8 reservoir can be divided into four types: type I of high quality reservoir,type II of preferably reservoir,III middle class reservoir and IV class poor reservoir.