Sedimentary facies characteristics and its mode of lower member of Oligocene Huagang formation in Xihu sag

The sedimentary facies of Xihu sag in Donghai shelf basin has been controversial.Based on core observation,combined with regional geology,paleontology,logging and seismic data,a comprehensive analysis of lower member of Oligocene Huagang formation in Xihu sag,such as sedimentary facies types,characteristics and distribution laws have been discussed in detail.The studies indicate lower member ofHuagang formation mainly developed four types,namely meandering river fan,braided river fan,meandering river delta and braided river delta.In northwestern sag,meandering river fan developed.In northeastern sag,braided river sedimentary developed.In south-central sag,meandering river delta and braided river delta deposits widely developed.On the basis of planar distribution of sedimentary facies characteristics and in combination with regional structure features,the related sedimentary facies models have been established.