Tertiary Evolution of Upper Slope Canyons, Offshore Equatorial Guinea: Canyon Initiation, Growth and Abandonment Via Knickpoint Migration

Slope canyons in the Rio Muni Basin, offshore Equatorial Guinea, have persisted since the Late Cretaceous and contain prolific hydrocarbon reservoirs. Analysis of a 3-D high-resolution seismic-reflection survey from the area reveals an upper slope canyon system where individual canyons aggrade vertically from the Cretaceous reservoir level to the modern seafloor. The slope canyons display low sinuosities (e.g., 1.1) and maintain their spatial location through syn-depositional faulting.

The canyons are modified by both syn-depositional faulting and/or canyon wall slumping. Large (e.g., ~ 400 m vertical, 5 km lateral) canyon margin bounding faults exaggerate the canyon topography and cause the canyons to aggrade rather than migrate. The outcome is a remarkable 700 m of aggradation during the Neogene, with less than 500 m of lateral migration of the canyon axis. The high aggradation/migration ratio (i.e., greater than 1) might concentrate sandstone in significant packages of stacked pay; however, the extensive faulting could complicate reservoir continuity.

Upslope and across-slope knickpoint migration is a first order control on sediment input into the canyons. In at least two cases, canyon piracy (sediment capture or ‘theft' from one canyon into another due to the latter having a knickpoint further upslope) causes abandonment of a canyon that was active for more than 10 Ma. The consequent development of a new canyon is relatively rapid and marked by multiple straight, parallel gullies that merge into a low-sinuosity master canyon. Understanding the complicated evolution of & interaction between canyons as well as their subsequent abandonment is important for both sand (reservoir) prediction and analysis of compartmentalization due to syn-depositional and tectonic modification.