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The Tertiary Niger delta covers an area of about 75,000 sq km and is composed of an overall regressive clastic sequence which reaches a maximum thickness of 30,000 to 40,000 ft (9,000 to 12,000 m). The development of the delta has been dependent on the balance between the rate of sedimentation and the rate of subsidence. This balance and the resulting sedimentary patterns appear to have been influenced by the structural configuration and tectonics of the basement.
Structural analysis of the Tertiary overburden shows that individual fault blocks can be grouped into macrostructural and eventually megastructural units. Such megaunits are separate provinces with regard to time-stratigraphy, sedimentation, deformation, generation and migration of hydrocarbons, and hydrocarbon distribution. A recurrent pattern emerges in the distribution both of absolute volumes of hydrocarbons and the ratio of volume of gas-bearing reservoir rocks to the volume of oil-bearing reservoir rocks within megaunits and macrounits.
The maturity of potential source rock in a given fault trend was achieved when sedimentation had almost reached the present surface, and when the active depocenter had been advanced seaward by several trends. Thus, migration started when deposition, together with the intrinsically synsedimentary structural deformation, had almost come to a halt in that particular trend.
The source rocks of the Niger delta yield a light waxy paraffinic oil, which is transformed bacterially to a heavier nonwaxy crude at temperatures below 150 to 180°F (65 to 80°C). The coincidence of the boundary between transformed and unaltered oils, within a rather narrow temperature range on a delta-wide basis, implies that little or no subsidence with concomitant increase in geotemperature of the oil-bearing reservoirs has occurred after migration.
The conclusion that migration took place after the structural geometry of a given trend had been determined originates from several independent lines of evidence. The observed uneven distribution of oil and gas in the delta therefore cannot be explained in terms of the passage of the source rocks through the oil-generating zone into the gas-generating zone (oil and gas "kitchens," respectively), with early structures receiving mainly oil and late traps receiving mainly gas. Rather, the hydrocarbon distribution probably is the result of original heterogeneity of the source rock and of segregation during migration and remigration.
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