Print this pageThe 3-D Seismic Geomorphology of a Deepwater Slope Canyon 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 3-D seismic data. We deal with the detailed seismic geomorphology of the Sequoia channel system, focusing on the geometry and distribution of its component sandbodies, their 3-D evolution in response to channel filling and the impact this has on reservoir heterogeneity.
The reservoir stratigraphy comprises a heterogeneous succession of sandstones and mudstones organized into a composite upward-fining profile. Component sandbodies are dominated by laterally amalgamated channels, sinuous channels and channel with frontal splays, and are interpreted to be the products of deepwater, gravity-flow processes. Above a basal incision surface, the reservoir is highly sand-prone and comprises 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 frontal splay elements. Shale and thin-bedded facies become an increasingly important component of the stratigraphy in the upper parts of the reservoir. The main channel is buried by a prograding slope succession that includes lobate sandsheets. The stacking of facies within the Sequoia channel system implies a punctuated waning of sediment supply prior to eventual abandonment.
The channel system also shows considerable evidence for syn-sedimentary faulting, including a large-scale, down-dip widening of the channel across a field-traversing flexure, small-scale channel diversions around a fault-tip and intra-slope ponding of flows on a footwall high.
Sequoia has the geometry, dimensions and internal sandbody organization that are consistent with the infilling of a 3rd-order lowstand channel incision. The channel fill culminates in a blanketing shale unit which delineates a major correlatable hot shale event, and on seismic data corresponds to a prominent down-lap surface (candidate maximum flooding surface).
Given the vertical variability in reservoir quality, understanding reservoir architecture in terms of sandbody geometries and connectivity is vital since across most of the field the gas column occupies the most complex and heterogeneous part of the reservoir. Correspondingly, the basal sand-rich part of the reservoir is likely to significantly influence aquifer behaviour during production.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009