Enterprise Oil is selling its 18.5 per cent interest in Ninian for dollars 75m to Chevron, Oryx and Murphy Petroleum, all of which already own substantial stakes in the field.
In a separate move Clyde Petroleum is disposing of its 1 per cent stake in Forties for pounds 5m.
Forties and Ninian are among the three biggest oilfields discovered in the British sector of the North Sea. However, their output has been in steady decline as their reserves are depleted.
Although they will continue production into the next century, they are relatively more expensive to operate than a new generation of smaller fields in the area.
Both sellers said they hoped to redeploy the proceeds to finance exploration and acquire more promising assets.
Roy Franklin, Clyde's managing director for the UK, said: 'Compared with Clyde's other producing interests, Forties are low-margin barrels. With declining rates of production, a disposal on these terms is attractive to us at this stage of the field's life.'
Chevron, Ninian's operator, is acquiring 6.53 per cent from Enterprise, boosting its holding to nearly 24 per cent, while Oryx's and Murphy Petroleum's stakes will go up to almost 30 and 14 per cent.
Ninian's total commercial reserves are estimated at 2 billion barrels, most of which has been tapped since production began in 1978. Daily production has fallen from 300,000 barrels in the early 1980s to about 65,000.
However, Chevron believes the field has considerable potential as it has another 1 billion of reserves that could become viable in future. The field also contains an extensive pipeline network used to transport crude oil from other areas.
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