| In recent years,the study of deep-water fine-grained sedimentary facies has always been a hot and difficult point in shale oil and gas exploration.Taking Chang 7 member of Yanchang Formation in Tongchuan area of Ordos Basin as an example,this paper has carried out the study of deep-water fine-grained rock in continental lake basin by using experimental means such as thin section observation,analysis of particle size,XRD,major and trace elements and rare earth elements analysis,which will provide scientific basis for the exploration and development of Yanchang Formation.Through lithology identification according to mineral composition and particle size,the fine-grained sedimentary rocks of Chang 7 member of Yanchang Formation in Tongchuan area were divided into siltstone,argillaceous siltstone,silty mudstone,mudstone and shale,and the logging response standard is established.On this basis,through a large number of field outcrops,samples,cores and thin section observation,the lithologic division scheme of silica,clay minerals and carbonate minerals was established,and the types of deep-water fine-grained sedimentary lithofacies were divided in combination with organic matter abundance and sedimentary structure.The deep-water fine-grained sedimentary lithofacies of Chang 7 member were divided into 10 categories.The lithofacies sequences of Tongchuan section,Yishicun section and Niejiahe section were analyzed,and six lithofacies assemblages dominated by gravity flow,deep lake and semi-deep lake were established.Through the analysis of various geochemical experiments,mainly including restoration of paleosalinity by Sr /Ba,judgment of paleoclimate by Sr / Cu,judgment of redox environment by V / V + Ni and U /Th,the paleoenvironment of the main types of lithofacies assemblages was analyzed.It shows that the environment from Chang 71 to Chang 73 in the study area is from semi reduction oxygenation to reduction,from semi saline to freshwater,and from semi humid to warm humid.On this basis,a deep-water fine-grained sedimentary model with deep-semi deep lake and gravity flow as the main sedimentary types was established,which was composed of three main lithofacies: tuffaceous facies,silty sandstone facies and shale facies. |
