Searching High and Low: Correlating Shallow and Deep Structural Trends along the West African Margin to Determine Sediment Transport and Hydrocarbon Migration Controls

Dickson, William ¹; Schiefelbein, Craig F.²
(1) DIGs, Houston, TX. (2) Geochemical Solutions International, Houston, TX.

Petroleum exploration probes the sedimentary record in ever more complex settings, straining our ability to define sediment delivery systems and predict hydrocarbon distribution. We present three examples of basement features controlling intra-sedimentary structuring and relate two outcomes to surface geochemistry to map inferred petroleum systems.

We determined basin frameworks with potential fields data; then correlated specific features to detailed published interpretations within the objective sedimentary section. Hydrocarbon leakage pathways to surface, confirmed via piston cores and SAR slicks, correlated to potential source rocks defined from crude oils analysis. Petroleum systems thus defined were mapped to structural control.

Niger Delta

Publications based on 3D seismic show precise terminations and offsets of intra-Miocene toe thrusts directly overlying the Chain & Charcot Fracture Zones (FZ) as defined on gravity imagery. (The remarkable proximity of +/-1 km in map view, within our nominal 5-6 km gravity resolution, results from FZ continuity across many image pixels.) These basin-forming FZ not only provide controls on initial sediment distribution, their influence persists. The intra-Miocene structures control younger drainage that steps around the ridges and follows accommodation zones across the affected area. Hydrocarbon leakage to surface is only detected where FZ intersect the main bounding toe-thrust belt. We infer this leakage produced BSR areas of gas hydrates described in the literature.

Lower Congo Basin

Carbonate banks, rafts and pre-salt highs correlate with gravity (responding to dense carbonates) and magnetic (more basement-sensitive) coverages. Surface geochemistry relates updip to carbonate bank and raft edges with associated listric faulting. Downdip, shallow salt and toe-thrusting reduce overburden sealing capacity allowing our best-defined seepage (piston cores, SAR slicks, sea-surface sampling) anomalies.

Sierra Leone-Liberia (SLL)

Published fan locations directly correlate with basement hinges and ramps, particularly where FZ intersect the SLL continental margin.

Three examples feature a passive margin with salt (Lower Congo Basin) and without (SLL) and a transform/passive margin (Northern Niger Delta) without salt. Studies of other margin basins will reveal similar controls and interactions, illuminating our understanding of exploration targets and petroleum systems.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.