The spokeswoman did not say whether oil production had stopped, but it is routine practice to shut down a facility when it is attacked.
The attack was apparently in response to the killing of eight people by troops guarding Ogbainbiri last week, security sources said.
The military said the dead were fighters who had tried to attack the oilfield, but a local group said they were mostly unarmed civilians.
Last week, Mujahid Dokubo-Asari, a prominent regional leader, was released on bail meeting a one of the main demands of the armed groups who have led a campaign of kidnapping and bombings at oil facilities.
Meanwhile, the armed groups have released about 30 hostages since Yar'Adua's inauguration on May 29.
The groups complain of neglect and poverty in the Niger delta, which produces all of Nigeria's oil but sees few of the benefits. Many people in the region live without access to electricity, clean water, roads or decent schools.
Their demands range from greater local control over the oil revenues to outright independence.