乐东–陵水凹陷梅山组海底扇识别及沉积模式

Identification of submarine fan in the Meishan Formation of the Ledong-Lingshui Sag in the Qiongdongnan Basin and its depositional model

  • 摘要: 近年来,乐东–陵水凹陷新近系岩性圈闭成为南海海域勘探的热点,发现了多个大气田,且已发现的岩性圈闭均与海底扇、浊积水道密切相关,为了获得更多天然气发现,提高岩性圈闭的勘探成功率,应加强海底扇识别及沉积模式研究。本次研究基于乐东–陵水凹陷的钻井和三维地震资料,以经典层序地层学为指导,从钻井取心入手,综合运用测井、FMI成像测井及分析化验等资料,分析了乐东–陵水凹陷梅山组海底扇的岩相、沉积构造、测井相及地震相特征。梅山组海底扇岩性以粉砂岩为主,发育块状构造、小型交错层理和冲刷面,结合粒度、C-M图分析认为其为浊流沉积,后期受牵引流改造。在梅山组地层内识别出三种类型海底扇:水道化型、盆底扇型、底流改造型海底扇,可进一步划分为3个期次,各期海底扇形成演化、主物源方向受古地貌影响由陵水凹陷向乐东凹陷迁移,扇体规模逐渐变大。综合考虑坡折带、相对海平面变化、三角洲物源供给对沉积的控制,建立了乐东–陵水凹陷梅山组“源–沟–坡–扇”的海底扇沉积模式。

     

    Abstract: The Neogene lithologic traps in the Ledong-Lingshui Sag have become hotspots for exploration of oil and gas in the South China Sea in recent years. Some large gas fields are discovered closely related to submarine fans and turbidite deposits. In order to obtain more natural gas discoveries to enhance the exploration success rate in the lithologic traps, it has become an urgent task to researchers to identify the submarine fans and to establish their depositional model. With the support of drilling and 3D seismic data from the Ledong-Lingshui Sag and following the principles of sequence stratigraphy, we started researches on the cores from drilling holes. Lithofacies, logging facies and seismic facies as well as sedimentary structures of the Meishan Formation submarine fans in the Ledong-Lingshui sag are carefully studied using well logging, FMI imaging logging and chemical data as tools. The submarine fans of the Meishan Formation is dominated by massive siltstone, with small cross bedding and scouring surfaces, which is recognized as turbidity current deposits later modified by tractive current based on grain size distribution pattern such as C-M diagram. Three types of submarine fans, say, channel type, basin floor type, and underflow reformed type, are defined. The deposits may be further divided into three periods, and the formation and evolution of the submarine fans in each period as well as the main source direction are affected by paleotopograpic framework. With the fans moving from the Lingshui sag to the Ledong sag, the scale of fans increased gradually. Upon the consideration of the type and distribution of slope breaks, relative sea level changes, deltaic source supply, and paleogeomorphological control over sedimentation, a depositional model including source, channel, slope and fan is established for the Meishan Formation in the Ledong-Lingshui Sag.

     

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