Anglo-Dutch giant Shell says it has begun pumping oil from a huge new field off the Nigerian coast, boosting the country's total production by 10%.
Shell has had a strong presence in Nigeria for decades
Production had begun at the Bonga deepwater oil and gas field, and will produce 225,000 barrels per day.
However, delays and cost overruns mean the development cost $3.6bn (£2.1bn), up from original estimates of $2.7bn.
Shell's partners in Bonga include France's Total, US major ExxonMobil and Italy's Eni.
Nigeria is already Africa's biggest oil exporter and produces more than 2.5 million barrels per day.
The government hopes to increase production to four million barrels per day by 2010.
The new 60 sq-km field is situated 120 km offshore, in water depths of more than 1,000 metres.
"The target is to attain the current nameplate production of 225,000 barrels of oil and 150 million cubic feet of gas per day as soon as possible," Shell said in a statement.
Shell, the world's third-largest listed oil firm by market value, has stumbled financially in bringing some other large projects to fruition.
The budget for its Sakhalin 2 project off Russia's east coast was doubled to $20bn this summer, and some experts believe it could still cost more.