Restoring the area from oil spills could involve the world’s "most wide-ranging and long-term oil cleanup,” the United Nations Environmental Program report says.
Waterways in the oil-producing Niger Delta — the world’s third-largest wetland — have been left environmentally devastated from more than 6,800 recorded oil spills and more than 50 years of oil operations.
The area’s mangroves and fish-rich creeks have become one of the most polluted places on Earth, CNN says.
But the report, commissioned by the Nigerian government and funded by Shell, indicates that the heavy pollution is even more extensive than previously thought.
The release of the report comes shortly after the Shell oil company admitted liability for two oil spills, in 2008 and 2009. The Bodo people, a traditional fishing community who live in the Niger Delta, are suing Shell for millions of dollars in damages.
"We will study the [report] contents carefully and will comment further once we have done so," a Shell spokesman told CNN.
Ogoni communities have long complained about the damage that oil has caused, but have mostly been ignored, the BBC says.
The issue was highlighted by the writer and activist Ken Saro-Wiwa, who was executed in 1995 by Nigeria's military government in a move that received widespread international condemnation.
Shell last year paid $15.5 million in an out-of-court settlement in a civil case brought by members of Saro-Wiwa's family and other supporters, CNN says.