LAGOS, Nigeria — Oil from an offshore spill has spread roughly 100 nautical miles after a leak occurred while loading a tanker, a Nigerian official said Thursday.

Royal Dutch Shell shut down a deep-water oil field off Nigeria's oil-rich southern delta on Tuesday.

Peter Idabor, the head of the country's oil spill management agency, said the offshore oil spill is likely the worst in a decade.

He expects oil to begin washing ashore on Nigeria's southern coast later Thursday.

Shell announced Wednesday it had closed its Bonga field after a leak of less than 40,000 barrels of oil that occurred while transferring crude oil to a tanker. The firm said it is cleaning up the spill.

'Natural dispersion and evaporation'
In a statement, Shell said that "up to 50 per cent of the leaked oil has already dissipated due to natural dispersion and evaporation."

It characterized the area covered by sheen as "large," but added that the sheen was "very thin in most areas."

Mutiu Sunmonu, Shell's country chairman in Nigeria, apologized for the incident.

"We are currently working with the Nigerian government to inform local communities and fishermen about the situation," he added.

Nigeria is a top supplier of crude to the U.S.

The company's website says Bonga, located 75 miles off the region's coast, has the capacity to produce more than 200,000 barrels a day of crude oil and 150 million cubic feet of gas a day.

Shell's pipelines in Nigeria's onshore Niger delta have spilled several times, which the company blames on sabotage attacks and oil theft.

said it will take as much as 30 years to clean parts of Nigeria's oil-stained Niger Delta.