Environmental Impact Assessment of Offshore Oil and Gas Operations on Marine Ecosystems in the Niger Delta

Offshore oil and gas production in the Niger Delta has transformed Nigeria's economy while simultaneously exposing marine ecosystems to chronic and acute disturbances. This study conducted an environmental impact assessment of offshore operations on marine ecosystems in the Niger Delta region, focusing on impacts from drilling discharges, produced water, seismic surveys, and accidental spills. Secondary data were compiled from environmental impact assessment reports of major oil companies, the Nigerian Environmental and Spatial Information System, and peer-reviewed literature from 2019 to 2024. Field surveys and biological sampling were conducted at three offshore stations at depths ranging from 50 to 200 metres. Macro-benthic community structure, fish diversity, and water and sediment quality were assessed and compared against unimpacted reference stations. The assessment identified produced water containing total petroleum hydrocarbons, drilling muds containing barite, and chronic seabed disturbance from drill cuttings as primary sources of benthic community degradation. Marine mammal disturbance from seismic air guns was identified as an emerging area of concern. Produced water hydrocarbon plumes extended up to 2.4 kilometres from discharge points in surface waters. Macro-benthic diversity was significantly lower within a 500-metre radius of active wellheads. The study concludes that current mitigation measures specified in EIA approvals are inadequate and recommends a transition to zero discharge of drill cuttings, mandatory pre-operational baseline surveys, and independent post-operation biological monitoring. Keywords: environmental impact assessment, offshore oil, Niger Delta, benthic communities, produced water.