Geological Characteristics And Enrichment Law Of Chang 8 Reservoir In South Ansai,Ordos Basin

Tight oil in Ordos Basin is widely distributed and has large reserves.With the increase of exploration and development,the proportion of tight oil's output increases year by year.However,due to the poor physical properties,strong heterogeneity and great changes in oil saturation of low permeability reservoirs,there are different understandings and opinions on the controlling factors of reservoir formation and enrichment laws in tight oil,thus affecting the exploration and development ideas and methods in tight oil.The main horizon of early exploration and development in Ansai Oilfield is Chang 6 and above,and the research on reservoir-forming geological conditions and enrichment law of Chang 8 oil layer is relatively weak.The existing research results show that Chang 8 reservoir has poor physical properties and belongs to low permeability and ultra-low permeability reservoir.From the perspective of oil and gas accumulation,the study area deviates from the hydrocarbon generation center,oil and gas migrate laterally,and oil sources are insufficient,resulting in insufficient filling of Chang 8 reservoir;The characteristics of source rocks,sediments,reservoirs and seepage flow that control the enrichment of Chang 8 reservoir are unclear,and the difficulty of exploration and development is gradually increasing In this paper,Chang 8 oil-bearing formation in Ansai South Area of Ordos Basin is taken as the main research object.On the basis of core observation,the reservoir characteristics of Chang 8oil-bearing formation in the south of Ansai Oilfield are studied in detail,and the oil and gas distribution law and main controlling factors are discussed from the aspects of sedimentation,diagenesis and structure.Chang 8 reservoir group in the study area belongs to delta front subfacies deposition,and the underwater distributary channel sand bodies are dominant aggregates,which are distributed in the NE-SW direction.The reservoir rock types are mainly fine feldspar sandstone and feldspar lithic sandstone,with an average interstitial content of13%,mainly iron calcite,chlorite and hydromica.The pore types are mainly intergranular pores and feldspar dissolved pores,with an average pore ratio of 3.3%.The reservoir physical properties are mainly ultra-low permeability reservoirs.The comprehensive evaluation of Chang 8 reservoir in the study area is divided into 4 categories and 5 sub-categories,among which I and II reservoirs have high oil saturation and good oil testing effect,and are comprehensively evaluated as high-quality reservoirs.The main factors affecting the reservoir enrichment in this area are oil source,sedimentation,reservoir and structure.Chang-8reservoir has good oil source conditions.Chang-7 and Chang-9 source rocks supply hydrocarbon bidirectionally,and lithologic reservoirs are mainly developed.Microstructures have limited control over reservoirs.According to the analysis of reservoir-forming factors,based on sedimentary microfacies,sand body distribution,logging interpretation and production test,and based on the comprehensive evaluation results of reservoirs,the favorable areas are predicted in Chang 8₁¹,Chang 8₁²,Chang 8₂¹and Chang 8₂²respectively,and 5favorable target blocks,which are mainly distributed in southwest and southeast.