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Assuming a simple model of delta development involving progradation and uniform subsidence to present depths (rate, 500 m/m.y.; 1,640 ft/m.y.), oil-genesis nomographs derived from the TTI method were constructed for various geothermal gradients of the Niger delta (2.2., 2.5., 2.9, 3.2, 3.6, 4.0, 4.4, and 4.7°C/100 m) and utilized in mapping the positions (depth, temperature) of the top of the oil-generative window (OGW) at arbitrarily selected times (40 m.y.B.P., 30 m.y.B.P., 15 m.y.B.P., and the present). About 200 data points were evaluated.
During the active subsidence phase, oil generation within any megastructure was initiated at a temperature of 140 to 146°C (284 to 294°F) and depth of 3,000 to 5,200 m (9,842 to 17,060 ft) within 7 to 11 m.y. after deposition of the potential source rocks. After cessation of subsidence, upward movement of the OGW by 800 to 1,600 m (2,624 to 5,249 ft) was accompanied by a temperature lowering of 23 to 54°C (73 to 129°F). Lower temperatures produced correspondingly heavier crudes.
In some parts of the delta oil generation and expulsion from the lower part of the Agbada Formation predates the cessation of subsidence and structural deformation, while in others it postdates those events. In most parts of the Niger delta, the upper and normally compacted part of the Akata Formation appears to constitute the major source rock.
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