12/26/2011 02:16 EST | Updated 02/25/2012 05:12 EST

Royal Dutch Shell says worst Nigeria offshore oil spill in more than a decade now contained

Officials with Royal Dutch Shell PLC say they've contained the worst Nigeria offshore oil spill in more than a decade.

Shell officials said Monday that the Dec. 20 spill that saw less than 40,000 barrels of oil pouring into the Atlantic Ocean has been dispersed. Shell officials acknowledged those aboard the Bonga vessel only noticed the spill after seeing it in the morning light, likely hours after it began leaking.

Shell said an investigation into the spill is ongoing.

Production at the field, representing about 10 per cent of Nigeria's oil output, remains shut in.

Shell operates the field in partnership with Italy's Eni SpA, Exxon Mobil Corp., France's Total SA and the state-run Nigerian National Petroleum Corp.