The levels of biodegradation of five whole oil samples from five oil fields in the Niger Delta were assessed using a high-resolution gas chromatography. The results show that all the samples are at different levels of degradation ranging from slight to heavy. The crude oils from Fs, Bs and Qe fields had undergone extensive to heavy degradation, while those from Ai and Ea fields had undergone slight to moderate degradation. The low paraffinic content values in the samples suggest that the nparaffins have been degraded and reduced consistently. The aromatic fractions (C 14 -C 18 ) have been reduced slightly, though the 3-Methylhexane component was observed to show strong resistance to the bacterial attack even at deeper fields. These levels of biodegradation show indications that they could be caused by the degrading anaerobic bacteria present in the deep reservoir and/or by the influx of meteoric water, especially for the Bs crude, which is from an onshore field. As this degradation continues to deplete the light hydrocarbon components, the residual heavy components such as sulfur and asphaltenes are gradually concentrated, while the API gravity is consequently lowered.
Basin modelling, constrained by geochemical data from eight wells, was carried out, across the late Cretaceous and Early Palaeogene sediment packages of southeastern Nigeria sedimentary basins. The study was aimed at establishing the sediment burial history, thermal maturation of the source rock and timing of hydrocarbon generation. The Late Cenomanian to Early Turonian Lokpanta Shale, which is the basal unit of the Eze-Aku Formation in the Eze-Aku Group, consists of alternating dark grey to black shales, marl and siltstones. The upper sections are mainly alternating sandstones, shales and limestones. This unit is the key petroleum source rock for the basin. Numerical 1D basin model of the study area revealed that the Cenomanian to Turonian times was the main phase of rifting in the Benue Trough evidenced by rapid subsidence. Subsidence rates varied widely in all the wells studied, ranging from 29-61 m/Mya (million years ago) and averaging 44 m/Mya. Subsidence rates also varied widely through geological time from the Cretaceous to the Palaeogene. The Cenomanian-Turonian and the Maastrichtian ages recorded the highest subsidence rates (169.75 and 168.28 m/Mya, respectively). These phases of rapid subsidence correspond to the main phase of rifting (Cenomanian-Turonian) and periods of increased sediment supply (Campanian and Maastrichtian), due to rapid erosion and unroofing of the structurally inverted Benue Trough, post-Santonian. Vitrinite reflectance values (1.87-4.78% Ro) indicated that the Lokpanta source rock (Late Cenomanian-Early Turonian) is mature to overmature. The vitrinite maturation profiles and the geochemical data suggested the generation of hydrocarbon before the Santonian compressional uplift of the Abakaliki-Benue Trough with its resultant sediment folding, which displaced the depo-centre from the Abakaliki Basin to the Anambra and Afikpo platforms. The subsequent erosion and non-deposition in the Abakaliki Basin raised the Lokpanta Shale above the oil generative window. The renewed sedimentation in the Campanian resulted in sagging due to sediment load creating the Anambra Basin. The lack of an effective trapping mechanism in the pre-Santonian may imply that hydrocarbon generated before the uplift migrated away, probably to upper and/or lower horizons (observed as oil shows in the region); some of which may be contributing to the Niger Delta crude. This is evidenced by the correlation of the Niger Delta deep sea samples to the Cretaceous (Lokpanta Shale) source rock and occurrence of biomarkers of Cretaceous origin (ab-hopanes and oleananes in the Opuama Channel complex, northern depobelt, Niger Delta).
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