Abstract: 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.

Key words: East Africa, Rovuma Basin, ultradeep water, gravity flow sandbodies, reservoir characteristics, forming process, strong bottom current