Shell offshore Nigeria oil facility shut after leak

* No planned restart date for facility

* Shell says half of spilled oil has dissipated

LONDON Dec 22 (Reuters) - Royal Dutch Shell said on Thursday that its giant 200,000 barrel per day (bpd) deepwater Bonga facility is now shut, with no planned restart date, after an oil leak occurred on Tuesday while a tanker was being loaded.

The field pumps around 10 percent of crude oil from Africa's top exporter.

"It's too early to say when it will restart. The focus is on dispersing the spill," said a Shell spokesman by telephone.

Shell said earlier this week that "less than 40,000 barrels of oil" had leaked at the site, located about 120 km off the coast of Nigeria in the Gulf of Guinea.

Around half of the spilled oil has now dissipated due to natural dispersion and evaporation, Shell said in a statement on Thursday.

"To accelerate the clean-up at sea, we are deploying vessels with dispersants to break up the oil sheen at sea. We are mobilising airplanes that will support the vessels in this operation," said Shell's country chair in Nigeria, Mutiu Sunmonu.

The oil major's website has a series of photographs taken at the site, showing a rainbow-coloured oil sheen on the ocean's surface.

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