The Sedimentary System And Distribution Of Jurassic In Wudun Sag Of Dunhuang Basin

There exists very limited research about Wudun Sag in the previous study.With the breakthrough in Well Xican 1 for oil and gas exploration,we confirm that it has become extremely meaningful to conduct research on depositional systems and distribution pattern of the Dashankou Formation of Lower Jurassic and the 1st Member of Zhongjiangou Formation of Middle Jurassic.Field outcrop,well-logging data and 2D seismic data were used to study the spatial distribution characteristics of sedimentary facies and the favorable sandstone reservoir prediction of the target formation.The provenance of Dunhuang Basin is mainly from Mountain Bei,Mountain AlkinQilian and Mountain Sanwei.Mountain Sanwei provides coarse sediment to the north and south.Mountain Alkin provides the coarse sediment to the north,and Mountain Bei provides relatively fine-grained sediments to the south.There are two main sources of South and North,and there is one minor source of Eastern in Wudun Sag.Five main types of sedimentary facies in the Lower Middle Jurassic in Wudun Sag are braided river,braided river delta,shallow braided river delta,fan delta and lake.Braided river delta developed during SQI of Lower Jurassic in the north and east of Wudun sag.Shallow braided river delta developed during SQII of the Middle Jurassic in the north and north-east of Wudun sag due to the gentle slope gradient of lake basin,the abundant source and the shallow water.Fan delta developed during SQI~SQII in the south of Mountain Sanwei.In the Lower Middle Jurassic in Wudun Sag,more than five seismic facies are recognized,including parallel-subparallel,chaotic,foreset,lenticular and fill seismic reflection configuration.The distribution of sedimentary reservoir can be confirmed from Stratimagic wave classification technique.The horizontal and vertical variations of sandstone and mudstone in the study area are verified by using colored inversion with few well constraints.According to the results of seismic facies,sedimentary facies and seismic attributes in this thesis,the underwater distributary channel of braided river delta,shallow braided river delta front in the north and fan delta in the south are all the favorable sandstone reservoirs.