Stratigraphic Controls on West African Deepwater Reservoir Connectivity and Production Performance

Rob E. Hill
ExxonMobii Upstream Research Co, Houston, TX

Edop, Zafiro and Girassol fields are reservoired in deepwater continental slope channel systems fed by ancestral Niger and Congo rivers. The degree of erosion, channelization, and confinement of each system evolves through its depositional history in response to relative sea level change, seafloor topography, and active tectonism. Although the resulting reservoir facies and architecture share common characteristics, field-specific stratigraphic and structural controls produce important differences in reservoir connectivity and production behavior.

The complexity of connectivity pathways, vertical and lateral baffles and barriers, and the Kv/Kh characteristics of individual reservoir zones relate directly to the depositional architecture of channel-complex elements within the slope channel system. Component architectural elements include highly meandering channels with point bar-like lateral accretion packages, low-sinuosity channels, weakly confined channels, and sheet-like distributive sand bodies.

Systems dominated by weakly confined to distributive channel complexes typically possess excellent lateral connectivity extending pressure communication for distances of several kilometers or more. Minimal erosion and associated high preservation potential for interstratified shale layers in these aggradational deposits produces a low Kv/Kh ratio.

Lateral and vertical connectivity within confined slope-channel systems is much more complex and variable. Vertical and lateral baffles and barriers occur at different stratigraphic scales, and this together with the reservoir distribution can be best characterized in terms of the deepwater stratigraphic hierarchy (Sprague et al, 2003). Capturing the hierarchical stratigraphic elements that define connectivity and resulting pressure communication in the reservoir is a critical prerequisite to building geologic models that can produce accurate history matched or predictive simulation models.