Peter Idabor, head of the National Oil Spill Detection and Response Agency told the Associated Press that the slick from Shell’s Bonga field has affected 115 miles of ocean near Nigeria’s coast, and is approaching the southern shoreline.

Idabor said officials expect the slick to reach beaches in Rivers state by Thursday afternoon. The spill likely occurred as workers for major oil-producing company tried to offload oil onto a waiting tanker, the AP reported.

Idabor said that while the source of the leak has been plugged, the spill still proves a threat to the shoreline and wildlife of the region. The company claims that 50 per cent of the spill, which they said on Wednesday was likely less than 40,000 barrels or 1.68 million gallons, has evaporated.

Idabor said the spill is the “most major one” since the Mobil spill of 1998 and said experts from Britain were coming to help with the clean-up, according to the AP.

Slicks from the Bonga spill likely will reach beaches near Forcados on Thursday, affecting wildlife there, Idabor told the AP.