FACTBOX-Which oil firms are affected by Niger Delta unrest?

June 25 (Reuters) - Nigeria's main militant group said its fighters sabotaged a Royal Dutch Shell oil pipeline in the southern Niger Delta on Thursday, the latest in a string of attacks against Africa's biggest energy producer. Following are details on which oil firms have been affected by the unrest and what they have said about production:
CHEVRON
The U.S. energy firm said on May 25 it had shut in approximately 100,000 barrels per day (bpd) of oil production from its swamp operations in Delta state following the first major strike in the militants' latest campaign.
The Movement for the Emancipation of the Niger Delta (MEND) said that day it had sabotaged pipelines to flow stations at Alero Creek, Otunana, Abiteye, Makaraba and Dibi in Delta state.
MEND claimed on June 10 to have sabotaged Chevron's Otunana pumping station in Delta state but the military denied any such attack had occurred and that a fire at the facility, confirmed by Chevron, was a result of a systems failure.
On June 13, Chevron confirmed there was a breach on its Makaraba-Utonana-Abiteye pipeline and fire at its Makaraba Jacket 5 facility in Delta state but said output was unaffected as the infrastructure had already been shut down.
MEND said on June 15 it had attacked Chevron's Abiteye flow station.
ROYAL DUTCH SHELL
The Anglo-Dutch energy firm said on June 25 it had shut down one of its pipeline valves at Krakrama manifold in its eastern operations in the Niger Delta after MEND claimed an attack on the Billie/Krakama pipeline in Rivers state.
The firm declined to say whether output was affected.
Shell also said it was investigating reports of attacks at three locations on June 21, two of them in the eastern state of Rivers.
MEND said it had attacked Shell pipelines at Adamakiri and Kula, both in Rivers state, and had later attacked what it described as part of an offshore oilfield in shallow water further west, saying that the structure was "engulfed in fire".
A senior industry source said the third attack was not thought to have been on an offshore installation as MEND claimed, but on a facility located in the mangrove creeks in the same broad area as where the first two attacks took place.

Shell said on June 18 some oil production had been halted following an attack on the Trans Ramos pipeline the previous day at Aghoro-2 community in Bayelsa state.
On June 17, Shell extended a force majeure on its Nigerian Forcados oil shipments for the rest of June and all of July. The measure, which frees it from contractual obligations, was first imposed in March after an attack on its trans-Escravos pipeline.
AGIP
MEND said on June 19 it had attacked an oil and gas pipeline at Nembe creek in Bayelsa state operated by Italian energy firm Agip, which delivers oil to the Brass export terminal.
Agip said the attack on the pipeline operated by its Eni unit between the Ogoda Manifold and the Brass terminal halted production of around 33,000 bpd of oil and 2 million cubic metres of gas per day.
It said its equity share of the lost output was around 6,000 bpd of oil and 437,000 cubic metres of gas.
The Italian firm declared force majeure on crude oil exports from Brass River on June 23.