Sandstone Genesis In Chang 8₂ Formation Of The Longdong Area

Longdong region is located in the southwest of Ordos Basin.In Longdong area,the Chang8₂ reservoir layer of the Yanchang Formation of triassic system is an important stratum for exploration and evaluation,which is currently located in Zhuang Well No 9 and other five oil-enriched areas.In recent years,test oil of the wells like Luo 152 Well and Li 263 Well in Chang 8₂ reservoir have all obtained high-yield industrial oil,indicating a good prospect for exploration and development future in the Longdong region.However,with further exploration and development,the original research becomes difficult to meet the production requirements,and it is imperative to disentangle the causes of sand bodies in higher accuracy and to analyze the sand body distribution characteristics caused differently in order to guide the following exploration and development efficiently.On the base of the many previous researches,this paper comprehensively applied drilling,well logging,and test analysis data in the research.Through the sedimentary structure,lithology characteristics,and logging curve characteristics,the source direction and sedimentation of the Chang 8₂ oil reservoir group in Longdong area are studied.Sedimentary facies and sand body structure characteristics,and their causes are also studied in depth.Based on the analysis of the sedimentary sequence of the Yanchang formation in the Ordos Basin,it can be concluded that the background of Chang 8₂ depositional period is a shallow-water lake basin depositional,forming a shallow-water and delta-lake sedimentary system.With light/heavy minerals,skeleton minerals,paleocurrents and other data,the direction and scale of the provenances of the Chang 8 depositional period were analyzed-the Chang 8depositional sands from multiple sources developed into four major source systems as the southwest,south,northeast and the northwest,mainly in the southwest.The types of sedimentary microfacies were determined comprehensively according to cores,field outcrops,well facies identification,and analysis data of grain size.And the sedimentary microfacies partitioning system in the study area is established.According to the evidence from modern deltaic depositions,expanding characteristics of the Chang 8₂depositional facies development is confirmed----according to the migration characteristics of the lake shoreline,the sedimentary facies belts of the shallow water delta are divided into the delta upper plain,the delta lower plain,the delta front and prodelta,due to the absence of fixed lake shoreline in the shallow water delta.Sedimentary microfacies are mainly diverted rivers and estuary dams.The northeast source sediments and the western/southwest source sediments meet in the form of Hekou Dam,and the deposition center is located in the area of Huanxian-Huachi-Taierwan.The sand bodies of the river channel are distributed in strips,and the bifurcation intersects frequently near the shoreline,which erodes the sediments of the early river channel and the estuary dam.Through log facies analysis and sand thickness statistics,the Chang 8₂ sand body structure is anatomized,the characteristics of shoreline migration are clarified,and the mechanism and distribution law of the Chang 8₂ dominant sand bodies are clarified.The single sand body structure in the Chang 8₂ oil-bearing section is mainly composed of distributary channel sand body,river mouth bar sand body,and mat sand body.The type of composite sand body in the study area is mainly composed of superposed distributary channel,superimposed estuary dam,and residual observatory dam river,as well as the complete composition of the sand body in complex four-kinds sand structure.Among them,the multi-stage superimposed thick sand channel body,river channel,and dam composite sand body constitute the main thick layer sand body of the Chang 8₂ reservoir section.According to the distribution rules of the sand bodies in the study area,four major thick sand deposition areas can be divided,three of which are thick sand areas along the mean low water level line,showing an intermittent northwest-southeast trend.Due to the formation of estuary dams,near the shoreline,the rivers have different degrees of bifurcation and diversion at different locations on the plane,which are prone to overlap rivers in different periods and tend to form local thickness increases.