南海西南部北康盆地新生代沉积演化史

History of the Cenozoic sedimentary evolution of the Beikang Basin,southwestern South China Sea

北康盆地位于南沙地块的西南边缘,为南沙中部海域的新生代沉积盆地,具有良好的含油气潜力。近年广州海洋地质调查局在盆地六万多平方千米范围内完成了近20000km的地震测线,及相应的重力与磁力测量。本文在区域地质背景分析基础上,对盆地进行了地震地层对比和详细的地震相分析,划分了3个超层序、7个层序,对其中的5个层序进行了沉积相分析,并编制了沉积相平面图。晚始新世以前,北康盆地位于古南海西北缘,盆地西北部为陆相环境,东南部为滨-浅海环境。晚始新世—早渐新世早期,南沙地块从华南陆块裂离向南漂移,随古南海洋壳的被动消减和新南海的扩张,盆地水体加深,除西北尚有陆相沉积外,主要为海相环境。早渐新世后,南沙地块与婆罗洲地块拼贴,北康盆地逐渐处于浅海-半深海环境。与盆地沉积演化规律相对应,早第三纪发育的湖相和沼泽相泥岩及早中新世发育的浅海相泥岩为有利烃源岩,同期的三角洲相砂岩、滨海相砂岩及晚中新世发育的浊积岩为主要储层,区域性盖层为晚中新世以来的浅海-半深海相泥岩、砂质泥岩,局部性盖层有湖相泥岩、浅海相砂质泥岩、泥岩和灰质泥岩。

The Beikang Basin is an important Cenozoic sedimentary basin located at the southwestern mar-gin of the Nansha massif,in the middle part of the Nansha sea area.It has good petroleum potential.In recent years,the Guangzhou Marine Geological Survey has completed about 20,000km seismic profiles and related gravity and magnetic surveys in an area of more than60,000km 2 in the basin.On the basis of an analysis of the regional geological setting,the authors have carried out the correlation of seismic stra-ta and detailed seismic facies analysis,and then three super-sequences and seven sequences are distinguished.The sedimentary facies of five sequences among them have been analysed and their sedimentary facies maps drawn.Before the late Eocene,the Beikang basin was located at the edge of the Paleo-South China Sea,and its northwestern part was marked by continental facies and the southeastern part was marked by the littoral-neritic environment.In the late Eocene to the early part of the early Oligocene,the Nansha massif was separated from the South China continental block and drifted away to the south.Concomitantly with the passive subduction of oceanic crust of the Paleo-South China Sea and spreading of the Neo -South Chin Sea,the waters of the basin deepened.Then except in its northwestern part where there was conti-nental facies,the basin was mainly occupied by the marine environment.After the early Oligocene,the Nansha massif was amalgamated with the Borneo massif.Then the neritic-bathyal environment gradually predominated in the Beikang basin.Pursuant to the law of sedimentary evolution of the Beikang basin,Pa-leogene lacustrine and swampy mudstone and early Miocene neritic mudstone are favorable hydrogen source rocks,contemporaneous deltaic sandstone and littoral sandstone and late Miocene turbidites are main reser-voir rocks,the regional covers consist of neritic to bathyal mudstone and sandy mudstone developed since the late Miocene,and the local covers include lacustrine mudstone and neritic sandy mudstone,mudstone and lime mudstone.