Three-dimensional seismic geomorphology of a deep-water slope-channel system: The Sequoia field, offshore west Nile Delta, Egypt

Within the Nile Delta gas province, reservoirs are dominated by Pliocene slope-channel systems, which are spectacularly imaged on high-quality three-dimensional seismic data. This article deals with the detailed seismic geomorphology of the Sequoia channel system, focusing on the geometry and distribution of its component sand bodies and the impact they have on reservoir heterogeneity. The Sequoia reservoir serves as a potential analog for similar but less well-imaged, deep-water slope systems. The reservoir consists of a succession of sandstones and mudstones organized into a composite upward-fining profile. Sand bodies include laterally amalgamated channels, sinuous channels, channels with frontal splays, and leveed channels and are interpreted to be the products of deep-water gravity-flow processes. Above a major basal incision surface, the reservoir is highly sand prone and made up of laterally amalgamated channels. The medial section of the reservoir is more aggradational and exhibits laterally isolated and sinuous channels. Within the upper part of the reservoir, channels are smaller, straighter, and built of individual channels with associated frontal splay elements and less common leveed channels. The main channel system is buried by a prograding slope succession that includes lobate sand-sheet elements. The stacking of facies within the Sequoia channel system implies a punctuated waning of sediment supply prior to eventual abandonment. The Sequoia channel is interpreted to be the late lowstand to transgressive infilling of a third-order early lowstand slope incision. The channel fill is overlain by a mudstone unit, which delineates a major correlatable hot gamma-ray event, and on seismic data, is a prominent downlap surface and therefore a possible maximum flooding surface. The Sequoia channel system shows evidence for synsedimentary faulting, including a large-scale downdip widening of the channel and small-scale channel diversions and intraslope ponding of flows. Understanding reservoir architecture in terms of sandbody geometries and connectivity is vital within Sequoia because the gas column occupies the most complex and heterogeneous upper part of the reservoir. Correspondingly, the basal sand-rich part of the reservoir will significantly influence aquifer behavior during production.