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Assuming a simple model of delta development involving progradation and uniform burial at 500 m/m.y. (1,640 ft/m.y.) to present depths, oil-genesis nomographs derived from the time-temperature index (TTI) method were constructed for geothermal gradients ranging from 2.2° to 5.1°C/100 m (1.2°-2.8°F/100 ft) of the Niger delta and used in mapping the positions (depth, temperature) of the top of the oil-generative window (OGW) at various times between 40 m.y.B.P. and the present.
During the active subsidence phase, oil generation within any megasedimentary unit was initiated at a temperature of 140°-146°C (284°-294.8°F) and depth of 3,000-5,200 m (9,843-17,060 ft) within 7-11 m.y. after deposition of the potential source rocks. After cessation of subsidence, vertical upward movement of the OGW by 800-1,600 m (2,625-5,249 ft) was accompanied by a temperature lowering of 23°-54°C (41°-97°F), producing correspondingly heavier crudes.
In a central belt of the delta, hydrocarbon generation and expulsion from the lower part of the Agbada Formation predate the cessation of subsidence and structural deformation, whereas, in other areas, it postdates the cessation of subsidence and structural deformation. In this central belt, the Agbada is the major oil source, with the Akata serving as a gas source. In the other areas, both the Agbada and Akata constitute oil sources, which implies that the thermal conditions rather than the kerogen type influence the oil/gas mix in the Niger delta basin.
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