Significance of Fault Zones and API Gravities in Biodegradation of Crude Oils in Emeabiam-Apani-Okwukwe-Iloma-Iheoma-Jisike Parts of Eastern Niger Delta, Nigeria

The structural faults in this part of the Eastern Niger Delta has revealed four way dip closed anticline at both shallow and deeper levels, downthrown to the first major E-W trending structure, and showed that the fault zones in the near-subsurface were functional avenues for flushing freshwater sands of Benin Formation down to the underlying Oligocene reservoir sands. The freshwater oxidizes the occurring hydrocarbons in the presence of oil-eating bacteria in the fresh water habitat resulting in biodegradation of the crudes in the comparatively near-surface or shallow reservoirs. The consequent degraded crudes are highly viscous and of low API gravities. They comprise heavy crudes with emulsions, wax, and long-chain hydrocarbon crudes. These are indicators that the crudes have suffered biodegradation and characterized by their low API gravity values, occurrence of foamy patches, waxes, very viscous, thick crudes, sometimes condensates. The paleoenvironment of the host reservoir sands were traced to paralic, brackish to marine fluviatile, and fluvio-marine environments. Normal range of temperature values, normal to abnormal pressures, with invading Benin Formation freshwater occurrence enhanced the environment for biodegradation of hydrocarbons in the host reservoir rock. The occurrence of biodegraded crudes in reservoirs with invading fluids or freshwater flushing through the fault zones produced viscous and thick crudes of 8.3 API gravity in some fields, while it is dryness and emulsions in others such as Owu, Omerelu, Emeabiam and Ilomba fields. The challenge is the case of no sustained flow to the surface due to the biodegradation and subsequent low API gravities ranging from 8.3 API to 18.4 API in the Alaoma and Iheomafields. Ohaji South reservoirs, mostly about 34 API to 36 API gravity also has a deviation from good quality crudes in Ohaji South-4 ST4 which suffered biodegradation, with 18.4 API gravity of heavy crude.