As journalists looked on, a judge who mediated in the matter escorted the four men to the governor's office in southern Rivers State and turned them over to authorities. U.S. Embassy officials weren't immediately available for comment.
The men's identities were not released. Chevron Corp. reported May 9 only that the men had been seized when their work boat was stopped by gunmen armed with assault rifles and rocket-propelled grenades.
Some 200 foreigners have been kidnapped since armed militants stepped up attacks on the oil industry and the government in late 2005, pressing their demand that more oil revenue be put into the impoverished Niger Delta. Most hostages have been quickly released for ransom.
Growing lawlessness, including the armed attacks, in the oil-rich region have caused a 25 percent decrease in petroleum exports from Nigeria, Africa's leading oil producer.
In his inaugural speech Tuesday, President Umaru Yar'Adua appealed for an immediate end to violence in the region of swamps, creeks and mangrove forests. The largest militant group said it would consider the overture.
Elsewhere in the region, Royal Dutch Shell PLC said it had suspended oil shipments of 150,000 barrels a day because protesters had besieged one of its export terminals this week.
The protesters tampered with some pipeline equipment, forcing the shutdown of the Bomu Manifold center Monday in the Ogoni district of Nigeria's southern oil region, company spokesman Precious Okolobo said.
"We had to defer export of 150,000 barrels per day," Okolobo said.
It was the second disruption of supply on the pipeline, which sends oil from onshore facilities around the city of Port Harcourt to Shell's Bonny oil export terminal.