Gunmen had captured six Britons, one American and one Canadian from a rig 65 kilometres out to sea on Friday, in a sophisticated night-time attack by 20 to 30 gunmen in speedboats.
A deal was struck between the oil companies, the Bayelsa government and local communities in Nigeria before the release, regional government spokesman Ekiyor Welson told news agency AFP.
“The release was effected after the oil companies agreed to solve some welfare problems of the community: giving scholarships to students, providing employment and generally putting something back to the host community," he said.
The first men to be released from their ordeal were two Britons. Several hours later the other men emerged unharmed, local authorities said, "They are all safe and well," said Johnny Iganiwari, information commissioner for Bayelsa state in the Niger Delta, where the men were held captive in remote mangrove swamps.
The British embassy said that the men were being flown from the southern delta to Lagos, Nigeria's commercial capital, and would then leave the country.
The attack had no impact on oil output as the facility is an exploration rig that will not produce crude for years.
The militant Movement for the Emancipation of the Niger Delta (MEND), whose attacks have forced the closure of about 550,000 barrels per day of oil production since February, has said it was not involved in the latest abductions.
Common tactic
Kidnappings for ransoms are common in the Niger Delta, an impoverished wetland that produces the bulk of Nigeria's crude oil, but where locals have seen few benefits from the industry.
Oil companies deny paying money to secure the release of kidnapped staff, but security analysts say the companies usually do pay up and that these payments then encourage further abductions.
Attacks on oil facilities onshore or in shallow water are frequent in this region, but the raid on the Bulford Dolphin rig, located so far off shore appears to be unprecedented. Analysts say it places a question mark over the strategy of many oil companies which have been looking to avoid risks in the Niger Delta by turning to expensive deepwater drilling in the Gulf of Guinea.
Since January, 24 expatriates working for oil firms have been kidnapped and later released unharmed.