下刚果盆地早白垩世巴雷姆晚期深水重力流沉积的发现及意义

Discovery and significance of deep-water gravity-flow deposits of the late Barremian of Early Cretaceous in Lower Congo Basin

以下刚果盆地下白垩统巴雷姆阶Pointe Indienne组中段为例,采用深水沉积学理论,结合重力流的最新研究进展,利用研究区最新钻井岩心资料及分析测试数据,对其岩性、沉积构造类型和岩石学特征等进行了探讨分析,识别出研究区广泛存在重力流沉积,并总结了该重力流的深水沉积过程及其砂体发育概念模型。结果表明:重力流沉积以中-细砂岩为主,砂岩底部发育槽模、重荷模、火焰状构造及球枕构造,砂岩内部有正粒序层理、平行层理、爬升波纹层理、包卷层理、泄水构造及漂浮泥砾;重力流砂岩主要为长石岩屑质石英砂岩,分选差、次棱-次圆状,以悬浮搬运机制为主,砂岩粒度累积概率曲线有全悬浮总体的"一段式"和高悬浮总体的"两段式";可识别出滑动、滑塌、砂质碎屑流、泥质碎屑流和浊流5种重力流流体类型及其在垂向上的6种相序组合类型;研究区重力流砂体横向连续性较好,垂向叠置厚度较厚,可形成规模较大的油气藏。

Taking the middle Pointe Indienne Formation of the Lower Cretaceous Barremian in the Lower Congo Basin as an example,based on the theory of deep-water sedimentology,combined with the latest research progress of gravity-flow,and with the help of the latest drilling core data and its analyzing test results in the study area,the lithology,sedimentary structure types and petrological characteristics were investigated.It is found that gravity-flow deposits were widely developed in the middle Pointe Indienne Formation of the study area.The deposition process and the conceptual model of sand-body development for the deep-water gravity-flow have also been summarized.The gravity-flow deposits are mainly composed of midium-fine grained sandstone where the flute cast,load cast,flame structure,ball and pillow structure are occurred at the bottom and the normal graded bedding,parallel bedding,climbing ripple bedding,convolute bedding,water escape structure and floating mud gravel are occurred in the internal part.The gravity flow sandstones whose transportation mechanism are dominated by suspension transport are mainly feldspathic detrital quartz sandstones which are poorly sorted,sub angular and sub rounded.Grainsize accumulation probability curve are characterized by both"one-segment"which suggests total suspension population and"two-segment"which suggests high suspension population.Five fluid types of gravity-flows including,slump,sandy debris flow,muddy debris flow and turbidity can be identified,and six styles of facies combination in vertical direction have been recognized.The gravity-flow sand-body in the study area can form large-scale oil and gas reservoir due to the better lateral continuity and the thicker vertical stack thickness.