Production problems at a North Sea oil field are propping up prices in the region, which plays a central role in setting the cost of crude worldwide.

About 400,000 barrels a day of North Sea crude are expected to be delivered via BP PLC's (BP) Forties pipeline in June, down nearly 20% from an earlier estimate. Traders linked the reduced volume to ongoing problems at the Buzzard field, which has fallen short of its normal output of 200,000 barrels a day this year while operator Nexen Inc. (NXY) performs maintenance and starts up a new platform.

Unexpected declines in Buzzard's production tend to have far-reaching effects in the global oil market. The field's oil gets mixed into the Forties blend, which in turn is the biggest component of benchmark Brent crude, against which more than half of the world's oil supply is priced. Brent's chief rival, U.S. benchmark West Texas Intermediate crude, is suffering supply problems of its own, as a glut of oil at the contract's delivery point in Cushing, Okla., has depressed futures prices.

This week, amid a broad plunge in commodities markets, June Brent futures prices held up better than contracts to deliver oil later in the year. The growing premium for June futures created a trading opportunity for investors who anticipated Buzzard's supply problems.

For example, a trader who bought June Brent futures and simultaneously sold July futures on May 6 would have made a profit of 20 cents a barrel over the next week as the "spread" between the two contracts grew.

"Traders will play the time spreads because...they will have some power to affect them, especially if they know ahead of time that Forties is going to be affected," said Torbjorn Kjus, oil market analyst at DnB Nor in London.

Owners of shares in the U.S. Brent Oil Fund (BNO) could also benefit if Buzzard's problems persist. During the first week of June, the fund will sell its holdings in front-month oil futures and buy into the next contract month. If Forties volumes are still low, the fund would be able to buy its new positions at a discount.

-By Sarah Kent, Dow Jones Newswires; 4420-7842-9376; sarah.kent@dowjones.com