UPDATE 2-Exxon warning adds to Nigeria oil output problems

* Exxon calls force majeure on Nigeria's biggest oil grade

* Shell's force majeure on Forcados exports in place

* Raft of output problems hit Nigeria in recent weeks

By Joe Brock and Emma Farge

ABUJA/GENEVA, Nov 21 (Reuters) - ExxonMobil on Wednesday became the fourth oil major in a month to warn customers over delays to Nigerian oil and gas exports, adding to a raft of problems for Africa's biggest energy producer caused by oil spills, theft and flooding.

Royal Dutch Shell lifted on Wednesday its force majeure on Nigeria's benchmark Bonny Light crude oil exports, easing some of the supply concerns.

But three of Nigeria main oil grades; Qua Iboe, Brass River and Forcados are still under force majeure. These oil streams together account for around 700,000 barrels per day (bpd) or around a third of total Nigerian exports.

France's Total and Italian oil firm Eni have also declared force majeure, on gas and oil supplies, respectively.

Nigeria is among the world's top 10 crude oil exporters and usually ships around 2 million bpd, but a major fire caused by oil theft, Exxon's spill and flooding have severely hit output.

Oil traders said that loading delays are worsening and are now up to two weeks for some cargoes.

Exxon's Nigerian unit declared force majeure on Qua Iboe crude oil exports on Wednesday due to outages caused by a pipeline oil spill on Nov. 9, which witnesses said had spread 20 miles down the coastline.

"There are issues with two areas in a pipeline resulting in hydrocarbon release offshore," an oil trader said.

Mark Ward, the managing director of Exxon Mobil's local unit, has said a clean up had been mobilised, and he apologised to affected communities for the spill.

Shell's outages were prompted by a huge fire on an oil tanker being used to steal oil last month. The Anglo-Dutch major shut another pipeline in Imo River on Oct. 31 due to theft damage and deferred 25,000 bpd.

Oil theft is a major problem in the winding creeks and waterways of the Niger Delta, where it is easy to conceal boats and illegal refineries in the dense mangroves. Nigeria estimates around 150,000 bpd is stolen, much of which is sold abroad.

Total last week restarted production from its 90,000 bpd OML 58 block, which was shut down a month ago due to severe flooding in the Delta swamplands.

Eni declared force majeure earlier this month on Brass River oil loadings due to floods, causing cargo delays of eight to 10 days.