An oil spill at an ExxonMobil facility offshore from the oil-rich Niger Delta has spread more than 30km from its source, coating waters used by fishermen in a film of sludge.
The oil slick comes from a pipeline off the coast of Akwa Ibom state that Exxon had shut down because of a leak a week ago. The cause is unknown.
A company spokesperson said a clean-up had been mobilised and apologised to affected communities.
This latest incident comes on top of multiple production problems in Africa's biggest crude exporter, after other energy companies reported disruptions at onshore sites due to oil theft and Nigeria's worst flooding in 50 years, Reuters reports.
It was the second major oil spill near Exxon facilities in three months. At the end of August, an oil spill left a slick running for kilometres along the coast.
Oil spills are common in Nigeria, where enforcement of environmental regulations is lax and armed gangs frequently damage pipelines to steal crude.
Energy companies say thieves are responsible for most of the spills on shore.