Structural consequences of fluid overpressure and seepage forces in the outer thrust belt of the Niger Delta
Résumé
In the Niger Delta, which has prograded onto the African continental shelf and neighbouring oceanic crust since the Eocene, abundant thin-skinned structures provide evidence for slope instability. The structures have detached on shale of the Akata Fm. At the toe-of-slope, the outer thrust belts contain compressional structures. The surface slopes and apical angles of the thrust wedges are small. Also, fore-thrusts alternate with back-thrusts. This is evidence for a small resistance to basal slip and it implies high values of fluid overpressure. Thrust faults are planar through the overburden, but strongly curved beneath it. By analogy with physical models, we take this as evidence for normal fluid pressures through the overburden, and for a pressure ramp, inducing seepage forces, beneath it. The seismic velocity of the Akata Fm. beneath the thrust belts is abnormally slow. This probably results from overpressure. Fluid vents are abundant within hanging-wall anticlines. A large one at a culmination communicates from a fore-thrust to the surface. The contents are seismically transparent, whereas the host rock is well stratified. We attribute the vent to fluidization. A nearby deep well encountered normal fluid pressures throughout the Agbada Fm., but a strong overpressure ramp beneath it. We argue that the most likely mechanism for explaining the observations is chemical compaction as a result of hydrocarbon generation, and it is suggested that the thrust belts have advanced seaward by a feedback mechanism.