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Facies and Depositional Model of A Deepwater Fan in the Zhujiang Formation,Liwan 3-1 Gasfield,Baiyun Sag,Pearl River Mouth Basin

LI Yun1 ZHENG Rong-cai1 ZHU Guo-jin2 GAO Bo-yu2 HU Xiao-qing2(1.State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation,Chengdu University of Technology,Chengdu 610059; 2.Research Center,CNOOC,Beijing 100027)  
Based on detailed analysis of cores,logs,and seismic data,ten lithofacies were indentified,including mega-thick and thick massive sandstones,thick normal-graded sandstones,thick inverse-graded sandstones,parallel and tabular cross-stratified sandstones,slumped and deformed sandstones,thin sandstone interlayers,thin mudstone-sandstone interbeds,thick siltstones,thick mudstones,and deep-water limestones.They can be organized into five lithofacies associations.These are sandstone-dominated,mudstone-dominated,mudstone-sandstone interbed,siltstone and deep-water limestone associations.Based on the analyses of lithofacies and their associations,the depositional system of the reservoir in the Zhujiang Formation,Liwan 3-1 field is identified as submarine fan.Four microfacies are further recognized within the fan based on detailed analysis,including channels(including main channels in the inner fan and distributary channels in the middle fan),inter-channel,levee-overbank,outer fan and basin microfacies.Facies analysis allows to reconstruct the depositional model of the deepwater fan.During relative sea-level fall,a basin-floor fan formed as sediments from rivers or shelf margin deltas were further transported down slope and into the basin under gravity flows.During the ensuing relative sea-level rise,a slope fan was deposited as the system stepped back.The lower part of the Zhujiang Formation,mainly sandstone-dominated associations,is inner fan and middle-outer fan deposits of the basin-floor and the slope fan respectively.The upper part,mainly mudstone-dominated associations,siltstone associations,and deepwater associations are outer fan deposits of the slope fan and basin deposits.Along with the transgression,submarine fan was finally replaced by pelagic deposits.
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