Depositional Characteristics And Prediction Of High-quality Reservoir Of Paleogene Sandstone-interbed Shale In Dongying Depression

Shale oil resources are abundant in Dongying Sag of Bohai Bay Basin,which is the key object of oil and gas exploration and development.The early exploration proved that the sandstone-interbed shale oil had high yield of single well and great exploration potential,but there was no systematic study on its sedimentary characteristics,development law and reservoir characteristics in the early study.In this article,through high precision turning stratigraphic classification and correlation,sandstone-interbed shale classification,combined with core,thin section observation and analysis of data statistics.Establish the classification scheme of high-precision cyclic strata and sandstone-interbed shale in the lower member of Sha3 of Paleogene in Dongying sag,discusses the development regularities of type sandstone-interbed shale,made its sedimentary characteristics and reservoir characteristics clear.Research findings include:(1)By focusing on the well cyclestratigraphy analysis and well profile correlation,the lower member of Sha3 of Dongying sag is established 400 ka long eccentricity cycle 6 points,125 ka short eccentricity 17 points graded stratigraphic classification scheme,combined with the ancient environment restoration results has been clear about the maximum flood flooding surfaces in e4 short eccentricity cycle,e4 ～ e7 short eccentricity cycle as the main flood period,The short eccentricity cycle of e10 ～ e12 is subflood period.(2)The logging interpretation scale classification scheme of sandstone interbedded shale assemblage is proposed,which can be divided into three parts: sand poor assemblage,sand bearing assemblage and sand rich assemblage.(3)It is clear that the sandstones are mainly composed of tractive structural sandstones,followed by oxidizing exogenous shilly-pebbly sandstones and deformed bedded sandstones.Combined with the characteristics that the mudstones are mainly composed of laminate and bedded dark mudstones,it is proposed that the middle-deep water sandstones are mainly formed by flood gravity flow deposition in the shale system of Dongying Sag.(4)Through the study on the relative ratio and plane distribution of sandstone interlayer,it is clear that the sandstone type assemblage in the deep depression zone is enriched in flood period,and the sand-bearing and sand-rich type is the main type,while the sand-poor type or sand-free interlayer is mostly developed in non-flood period.(5)Through the analysis of reservoir characteristics,it is clear that the sandstone interlayer is dominated by fine-coarse-grained lithic arkose sandstone,with strong cementation and compaction but weak dissolution.Intergranular pores are developed,and the physical properties are characterized by low-medium porosity and ultra-low permeability,and the correlation between porosity and permeability is weak.(6)Combined with the study on the vertical development of shale assemblage by paleoenvironment,it is clear that the e4 and e5 short eccentricity cycles are the most favorable ones;The sand-rich shale has the best physical properties among different combinations,and the TOC of shale in Lijin area is the highest in plane.Accordingly,the favorable shale reservoir horizons and favorable zones are optimized.