FORTIES, the largest oilfield discovered in the UK North Sea, is to be sold by BP in a deal that marks a watershed in Britain’s declining offshore oil industry.

The giant oil reservoir and a package of smaller US fields is being acquired by Apache, an American company, for $1.3 billion (£813 million). BP is shedding the rapidly shrinking Forties field and some Gulf of Mexico assets as it prunes its portfolio of high-cost and low-return oilfields.

Discovered in October 1970 by the Sea Quest drilling platform, production was inaugurated five years later by the Queen. The field rapidly came to symbolise Britain’s North Sea oil boom. Within three years, Forties was pumping 500,000 barrels per day down a 170-kilometre pipeline to the company’s refinery at Grangemouth in Scotland.

Forties has delivered 2.5 billion barrels of oil over three decades but today its produces little more than 45,000 bpd. While output shrinks, the cost of running the mammoth offshore platforms remains.

Margins have also been hit by the Chancellor’s 10 per cent surtax on oil profits. BP is retaining the Forties pipeline which collects oil from adjacent fields. The additional tax burden will have weighed heavily in BP’s decision to sell Forties and analysts speculated that other mature North Sea assets could be put on the block. “Brent could be the next candidate,” said JJ Traynor, of Deutsche Bank, referring to the huge Shell/Esso field discovered a year after Forties.

The discovery of Forties and Prudhoe Bay in Alaska in 1969 rescued BP from disaster at a time when governments in the Middle East and Africa were nationalising oil assets. Today its output represents just 1 per cent of the company’s worldwide production.

Yesterday’s sale follows a comprehensive review by BP of its portfolio, a result of last year’s embarrassing failure to meet demanding oil production growth targets. The sale, which will cut production, signals a return to financial targets and the abandonment of growth for its own sake.

BP emphasised yesterday that it would seek improved financial returns and higher value growth. The company said: “This means disposing of asset that may be worth more to others than to us.”

The arrival of Apache, an independent US oil producer, will reassure the Government, which is hoping to secure new investors to the North Sea as the oil majors shift their focus to bigger prospects in Africa and Asia. Forties and the Gulf of Mexico package will increase Apache’s oil production by a third and its reserves by 233 million barrels.