Structural And Sedimentary Evolution Of Chang8 Oil-bearing Formation Of Yanchang Formation In Shangliyuan Area,Ordos Basin

In this paper,the Chang8 oil-bearing formation of Yanchang Formation in Shangliyuan area of Ordos Basin is taken as the research object.Based on the previous research studies,guided by the basic theories and methods related to sedimentology,the data of drilling and logging and core testing and analysis are fully utilized,study the sedimentary evolution and reservoir characteristics of Chang8reservoir formation,the following results are obtained:?1?The Chang8 reservoir in the study area is divided into two major cycles,each of which is about 40 m,which is Chang8₁ and Chang8₂ sub-sections.Depending on the characteristics of marker beds,sedimentary cycles,lithologic assemblages and electrical properties,the Chang8 reservoir group in the Shangliyuan area is further divided into five sub-layers:Chang8₁¹,Chang8₁²,Chang8₁³,Chang8₂¹ and Chang8₂².?2?The Late Triassic Chang8 oil-bearing formation was deposited in the early stage of lake-basin depression development.Its structure was stable and gradually strengthened,topography was relatively flat.Affected by the provenance in the southwest of the basin,typical shallow-water delta deposits were developed,which can be divided into delta inner front,delta outer front and some microfacies according to water depth.?3?The Chang8 strata in Shangliyuan area of Ordos Basin are mainly feldspar sandstone and feldspar lithic sandstone.The proportion of feldspar and lithic debris in the clastic composition is relatively high,and the clastic type is followed by metamorphosed lithic cuttings as the main igneous rock cuttings,sedimentary rock cuttings are rare.The pore types are mainly intergranular pores,followed by feldspar karst pore and debris karst pore,with a small amount of intergranular pore and micro-fracture.The reservoirs experienced compaction,cementation,metasomatism and dissolution,and are generally in the middle diagenetic A-B stage.The favorable diagenetic facies zones are mainly chlorite film-intergranular pore facies,illite cementation-intergranular pore facies and illite cementation-feldspar dissolution facies.?4?Through the study of the main controlling factors of reservoir development in Chang8 area of Shangliyuan,it is found that the reservoir materials are closely related to the sedimentary microfacies of the sand bodies in the study area,among which the distributary channel sand bodies are the best developed.The relationship between the content of filling material and the porosity is mirror negative correlation.The reservoirs have undergone different diagenesis,among which the decrease of reservoir porosity in the study area is mainly due to compaction,followed by cementation,metasomatism has little effect on reservoir physical properties,and dissolution has increased reservoir physical properties to a certain extent.