Developmental Characteristics And Forming Conditions Of The Large Submarine Fan In Deepwater Area Of Qiongdongnan Basin

Qiongdongnan Basin,bounded by Yinggehai Basin with NO.1 fault in the west,Zhusan Depression with Shenhu Uplift in the east,Hainan Island in the north,and Yongle Uplift in the south,is a gas-rich basin in the western part of the north continental shelf edge of South China Sea.In recent years,the cognition of the overall structure and stratigraphic sequence of the basin is more comprehensive due to the coverage of large three-dimensional seismic data and the improvement of seismic quality.The sedimentary laws and longitudinal evolution of Qiongdongnan Basin in different periods were analyzed systematically,and the regular changes of large submarine fans gravity flow reservoirs in key zones,along with the structural filling and evolution of the basin,the rise and fall of sea level and the different reservoir type and distribution,were studied.The sourcesink system of deep-water sedimentary system was mainly studied.Byrevising and improving the distribution maps of each third-order sequence sedimentary system of the late Qiongdongnan Basin,the distribution range of the late submarine fans were implemented.The results show that abundant source,the rise and fall of sea level,suitable ancient landform and certain tectonic activities are essential elements for the development of large reservoir in Qiongdongnan basin.The foreset characteristics of western continental slope are obvious,and Songtao-Baodao district is dominated by accretionary continental slopes,while Changchang depression developed regressive continental slope sequence.The typical sedimentary models of submarine fans can be divided into the “source-ditch-slope-fan”sedimentary model,the multi-point source sedimentary model,and the sedimentary model.The sedimentary system of large submarine fans can be divided into five periods in the longitudinal direction.In the second period of Sanya formation,the submarine fans developed in Ledong depression to Huaguang depression,Lingshui depression,Songnan depression and Changchang depression,and distributed dispersedly.In the first period of Sanya formation,large submarine fans were influenced by Hainan Island source and NO.2 fault in the north of Qiongdongnan basin,while influenced by southern uplift source in Changchang depression.In the period of Meishan formation,the submarine fans developed especially in Ledong-Lingshui depression.In the period of Huangliu formation,less submarine fans developed in Ledong depression to Baodao depression.In the period of Yinggehai formation,the restrictive submarine fans developed on the central canyon channel.In the horizontal direction,the submarine fans developed in Changchang depression in the eastern basin,and developed in Ledong depression in the western basin,while the No.2 fault mainly controls the distribution of submarine fans in the northern basin.