Variation in Tectonic Style Along the West African Margin and its Control on Sedimentation

Rosemary A. Edwards¹, Christian Berndt¹, Lindsay M. Parson¹, and Doug Evans^2
1 Southampton Oceanography Centre, University of Southampton, Southampton, United Kingdom
² WesternGeco, West Sussex, United Kingdom

The deep-water areas of rifted continental margins represent the current frontier for hydrocarbon exploration. While the rewards can be very great, there are considerable financial risks. To quantify the risks it is vital to understand the early history of continental break-up and basin formation. The location and nature of the ocean-continent transition, along-strike variations in thermal and extensional subsidence and syn- and post-rift igneous activity all affect hydrocarbon maturation.

A recent reinterpretation of the WesternGeco Round Africa (RAF) and Probe II seismic reflection surveys has produced a regional synthesis of the different styles of West African passive margins and an understanding of the controls on, and transition between, these different margin segment types. The western margins of Africa exhibit several different styles of passive margin; ranging from non-volcanic and volcanic rifted margins to strike-slip dominated transform margins. After breakup the original margin structures were modulated by (1) different amounts of sediment input, e.g. river deltas vs. starved margins, (2) subsurface sediment mobilization, e.g. halokinesis and fluid flow, and (3) post-rift tectonic events such as the collision between Africa and Eurasia.

Interpretation of the reflection seismic data, which provides images to at least the top of oceanic basement and the top pre-rift, has been fully integrated with potential field data and existing wide-angle seismic velocity fields to provide estimates of crustal thinning at key locations along the margin. This data integration allows us to identify along-margin variations and thus provides valuable constraints for petroleum plays in this region.