GEOTHERMAL GRADIENTS AND BURIAL HISTORY MODELLING IN PARTS OF THE EASTERN NIGER DELTA, NIGERIA
Reservoir and bottom hole temperatures from seventy wells in the Eastern Niger Delta suggests that two leg dogleg geothermal patterns characterize the geothermal gradients pattern of the Central Swamp and the Coastal Swamp in contrast to the single gradient patterns seen in the Shallow Offshore. In the shallow/continental sections in the Niger Delta, geothermal gradients vary between 10 – 18 o C/Km onshore, increasing to about 24 o C/Km seawards. In the deeper (marine/parallic) section, geothermal gradients vary between 18 – 45 oC/Km. The average geothermal gradient for the various depobelts is 19 oC/Km for the Central Swamp, 17oC/Km for the Coastal Swamp and 20oC/Km for the Shallow Offshore. Geothermal gradients in the Eastern Niger delta increase eastwards, northwards and seawards from the Coastal Swamp. Vertically, thermal gradients in the Niger Delta show a continuous and non-linear relationship with depth, increasing with diminishing sand percentages. As sand percentages decrease eastwards and seawards, thermal gradient increases. Thermal conductivies also decreases with depth from about 2.3 W/mK in the continental sands to 1.56 W/mK in the parallic and continuous shaly sections. Isothermals constructed at three depth levels: 1000m, 2000m, and 3000m shows that depressed temperatures occur in the western rand north central parts and elevated temperatures in the eastern and northern parts of the study area, respectively. Heat flow computed from 1 – D modelling software and calibrated against BHT and reservoir temperatures suggests heat flow variations in the Niger Delta to range from 29 – 55 mW/m2 (0.69 – 1.31 HFU) with an average value of 42.5 mW/m2 (1.00 HFU). Lower heat flows (< 40 mW/m2) occur in the western and north central parts of the parts of the study area, and is likely to be influenced by high sedimentation rates. Higher heat flows (40 – 55 mW/m2) occur in the eastern and northwestern parts of the study area. Radiogenic heat production from crustal rocks and shale’s may account for the heat flow in the east. Hydrothermal convection is likely to have elevated the heat flow in the northwest. The hydrocarbon maturity modelling results show vast differences in timing and levels of kerogen transformation into petroleum. Result suggests that the potential source rocks (Paleocene, Eocene, Oligocene and partially the Lower Miocene) have attained maturity status to generate hydrocarbons. The depth to the onset of the oil window decreases from the west to the east and to the northwest.
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