东非鲁武马盆地始新统超深水重力流砂岩储层特征及成因

鲁武马盆地古近系-新近系发育多套超深水、超大型、富含天然气藏的重力流沉积砂体。以始新统砂体为解剖对象，分析区内重力流砂岩储层特征及成因。结果表明砂体以巨厚层状产出于深海泥岩内部，并与周围泥岩截然接触，测井曲线表现出宏观均一性；岩心揭示此类巨厚砂体是由多期单砂体叠置而成，单砂体是由底部高密度颗粒流和顶部低密度浊流两部分组成，且经历过强底流改造。鲁武马河流三角洲强大物源供给决定了区内砂体分布面积和体积规模；深海滑塌、块体搬运等重力流沉积过程控制了沉积体粒序构造和内部结构；海底区域性强底流持续冲刷并携带走单砂体顶部细粒沉积物，残留了底部“干净”的中粗粒砂岩；多期沉积事件和频繁水道迁移决定了砂体纵、横向叠加展布，并最终形成了区内厚度巨大、岩性宏观均一且连通性极好的超大型深水重力流沉积砂岩储层。

Characteristics and the forming process of the Eocene ultra-deep-water gravity flow sandstone reservoir in the Rovuma Basin,East Afirca

A number of Paleogene Neogene large area gravity flow sand bodies, bearing super huge volume of gas reserve, were developed in the ultra deep water region in the Rovuma Basin, East Africa. By studying the Eocene sand body, we investigated the characteristics of the extraordinary sedimentary complex and its corresponding forming mechanisms in different scales, using high quality seabed 3D seismic imaging, well data and nearly one hundred meters of valuable conventional core. From the seismic imaging, it was found that these sand bodies are lenticular and wedge shaped, enclosed by thick and dark colored deep sea mud or shale. From the wire line logging data, an abrupt contact exists between sandstone bodies and their surrounding mud. And the sandstones always seem homogenous with excellent porosity, permeability and especially low content of shale in whole interval. However, the detailed observation of bottom hole core indicated these seemingly homogeneous and extraordinarily thick sand bodies belong to gravity flow deposit of mainly sand rich turbidite channel and lobe complex, and are composed of several comparable gravity flow deposit units or sub layers. Each unit or sub layer mainly consists of coarse grained sandstone in the lower part, and fine grained sandstone, silt or clay in the upper part. A clear boundary could be easily identified between any two units, and the common lack of fine grained deposits in upper layers indicates these sandstones were modified to various extents by a certain geological process. After a comprehensive study of the sandstone complexes, together with research on regional sedimentary environment, the formation mechanism of the sandstones can be summarized as the following, the Rovuma river delta directly supplied huge volume of sand rich sediments to the large area deposit complexes; the deep water gravity flow deposition and block transportation controlled their grained bedding and internal architecture, one of the most important features of gravity flow deposit. Then the bottom current blew away the upper fine grained suspension of shale and silt sparing the medium coarse grained deposits and formed the fascinating deep water sandstone complexes. Finally, multiple depositional events and channel lateral migration together contributed to the formation of the thick, homogeneous and mud free sandstone bodies of this unique type of excellent sandstone reservoir.