Research On Sedimentary Microfacies Of Organic-rich Shale Of Longmaxi Formation In Weiyuan-Dazu Area, Sichuan Basin

Shale sedimentary facies analysis is an indispensible part in the research of shale oil and gas exploration.Both domestic and foreign researchers have realized that precise division of the sedimentary microfacies can lay a solid foundation for selecting the exact areas of exploration.The paper focuses on Long11 subsection in Weryuan-Dazu area.On the basis of datas collected by drilling,logging,cores and relevant analysis,the paper conduct the formation division of the Long 11 subsection.The research on the division of shale sedimentary microfacies is also carried out based on a comprehensive study of lithology,facies division,sedimentary environment and structure as well as the biological development.In consideration of the lithology,electrical properties,paleontological characteristics and elemental geochemical characteristics,the lower part of the Longmaxi—Longl section is divided into two subsections:Long1 and Long12.The Long11 is further divided into four layers named a/b/c/d,each layer has been set an criteria for division.Layer a and c are mainly composed of hypoxic habitats graptolite-rich shale and silt shale rich in biogenic silica in transgression period.Layer b and d are composed of graptolite-containing and silt shale as well as graptolite-containing and calcium-containing silt shale in regression stage.Vertically from a to d,the content of biogenic silicon decreases while land-based silicon grows.Meanwhile,graptolites changes from akidograptus to demirastrites and spirograptusAccording to the lithology,depositional environment and paleontological characteristics,the deep-water shelf in Longl is further divided into six micro facies,figuring out that strong retention of hyposia graptolite-rich shale micro facies mainly scatters in layer a and c.Mediate retention dysaerobic graptolite-containing shale spreads in layer b,c,d.and mediate retention dysaerobic graptolite-containing and silicon-containing shale strong retention hyposia-dysaerobic shale located in d.retention dysaerobic siliceous and calcareous shale micro-facies located in Wufeng formation.By a comparative analysis,a and c are the most favorable partitions for sedimentary micro facies,the plane is mainly distributed in the south of the W202-W204 well area and the south of the W206-Zu201 well area.