Sedimentary Characteristics of Baoshi Formation of Lower-Middle Eocene in Xihu Sag of East China Sea Basin

Xihu Sag is located in the eastern of East China Sea Basin,and is one of the important petroleum production areas in offshore of China.A pelite-prone section of Lower-Middle Eocene under the main hydrocarbon reservoirs in Xihu Sag was drilled from well Y1 in 2001.The thickness of the section was 360 mand was named as Baoshi Formation,which was developed in the initial stage of the faulting period of Xihu Sag.Based on the data of core,logging,paleontology and seismic,the sedimentary settings and distribution of Baoshi Formation in Xihu Sag from well Y1 were analyzed.The results show that the sedimentary setting of Baoshi Formation in Xihu Sag is paralic lacustrine;the sedimentary facies including fluvial,delta,shoreshallow,half-deep lacustrine,fan delta,etc.,develop in Baoshi Formation;fluvial facies develops in the northern of the west slope,and is characterized by lower continuity,varying amplitude,sub-parallel sheet-like seismic facies;delta facies develops in the west slope and the transition zone of the west slope and the central deep sub-sag belt,and is characterized by wedgelike or filling foreset seismic facies;fan delta facies develops in the eastern steep slope of Xihu Sag,and is characterized by clutter wedge-like seismic facies;shore-shallow lacustrine faciesdevelops in most parts of Xihu Sag,and is characterized by middle-good continuity,middle amplitude and parallel to sub-parallel sheet-like seismic facies;half-deep lacustrine facies develops in the central deep sub-sag belt,and is characterized by good continuity and high amplitude sheet-like seismic facies.Based on the structure-sedimentary filling analysis of seismic profiles,two types of depositional filling styles including underwater-height and half-graben develop in Baoshi Formation of Xihu Sag.