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Whereas hydrocarbon potential studies of most parts of the Nigerian sedimentary regions have been documented, no similar assessment has yet been made for the outer continental shelf (OCS), slope and rise of the Niger delta. The shelf, slope, and rise of the delta are frontier areas in which no wells have been drilled to date.
For this broad evaluation of hydrocarbon potential of the deep-water Niger delta, six published seismic sections were reviewed in conjunction with available geological information and geophysical data.
On the basis of an assumed average velocity of 2.0 km/sec, the seismic data indicate a sedimentary thickness of about 2 km (1.3 mi) in the deeper portions of the rise. This thickness increases shoreward to more than 3 km (1.9 mi) at the foot of the continental slope and 5 to 7 km (3 to 4.3 mi) beneath the outer continental shelf.
The distal relationship of the OCS, shelf, and rise to the sedimentary discharge of the Niger delta have made these areas of essentially deep-water marine sediments. With the high organic carbon content commonly associated with these areas, the thick shales in this distal deltaic environment would be very rich in kerogen. Coarse-grained clastics are identifiable on the seismic sections by their distinct and continuous reflection character. These potential reservoirs are common and widespread on the shelf and rise. The reservoir rocks occur as deep-sea fans, turbidites, canyon fills, and as onlap fills between diapiric intrusions, and are enclosed by shale, thus providing favorable conditions for the formation of stratigraphic traps. The diapirism in the slope and outer shelf provides avorable conditions for structural traps.
The tectonic origin of the Niger delta area implies a history of initially high geothermal gradients which decreased with time as the margin moved farther away from the Mid-Atlantic Ridge. The geothermal gradient map of the Niger delta indicates higher values for the OCS relative to the onshore and shallow-water areas where rapid sedimentation has depressed the geothermal gradients. The general increase of geothermal gradients toward the mid-ocean ridges would also provide gradients much higher than the 1.8°F/100 ft of the shallow shelf and probably approaching the 3.0°F/100 ft of the Anambra basin region. These gradient ranges are sufficient to mature the kerogen of the oldest shale source rocks (more than 40 m.y.) and to generate hydrocarbons within even the thinnest (2 km 1.2 mi) sediment observed. In the areas of thin sediment, hydrocarbon potential would be appreciably increased in the vicinity of oceanic fracture zones where geothermal gradients are locally high. The hydrocarbon potential increases shoreward as sedimentary thickness increases and is very significant in the upper rise, the slope, and the OCS.
The decline in giant field discoveries in the Niger delta is a pointer to the depletion of Nigeria's onshore and shallow-water oil reserves. The continuing increase in world oil demand in the face of dwindling reserves and the steady improvement in deep-water drilling technology combine to make future petroleum exploration and production economically promising in Nigeria's high-potential deep-water frontier.
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