ANATOMY 101: DISSECTING A WEST AFRICAN CHANNEL-LEVEE SYSTEM USING 3-D SEISMIC METHODS

Dissection of the shallow buried Benin-major channel-levee system, located on the mid-slope of the western Niger Delta, reveals a high level of detail and architectural complexity never before seen. Three-dimensional seismic data reveal several architectural elements including inner levees, outer levees, an erosional fairway, channel-axis deposits, rotational slumps, and mass transport deposits. The system is characterized by a basal erosional fairway that is bordered by outer levees of varying thickness. Together these elements define the base and margins of the channel-belt, where channel deposits and inner levees are the dominant architectural elements. Vertical, sub-vertical, and lateral stacking patterns of sinuous and/or meandering channels create seismic facies that range from narrow to wide zones of HARs (high amplitude reflections) with chaotic to continuous and shingled to horizontal reflections. Some HARs appear as isolated or stacked asymmetric to symmetric u- and v-shaped reflections, referred to here as channel-forms.

The channel-belt evolves within the confines of the scalloped erosional fairway walls (flanked by outer levee), and is similar in morphology to meander-belts in fluvial systems, but has a greater component of vertical aggradation. Three distinct phases of channel development are recognized during its aggradational history, each corresponding to a different channel stacking architecture, planform geometry, and nature of terrace development, with important implications for reservoir architecture. In some cases, multiple phases of inner levee growth are observed, each closely linked to the channel migration and aggradation history. Channel sinuosity evolves dynamically, with some meander loops undergoing periods of accelerated meander growth at the same time that others show little lateral migration.