AMSTERDAM (AP) — Britain's Heritage Oil PLC says it has agreed to buy a 45 percent stake in a major oil field in Nigeria for $850 million.

Nigeria's national oil company will retain a 55 percent interest in the on-shore field in the Niger Delta. The sellers are Royal Dutch Shell, which owns a 30 percent stake, France's Total SA, with 10 percent, and Italy's Eni S.p.A., which owns 5 percent.

Heritage said Monday it is buying assets already producing 35,000 barrels per day and significant reserves and pipeline assets. The purchase is structured as a joint venture with a Nigerian company, Shoreline Power.

Shell, the largest foreign producer of oil in Nigeria, has gradually been reducing its exposure to the country, and preferentially developing offshore fields rather than on-shore ones.