A drop in supply of crude that underpins the Brent crude contract in September has the potential to boost prices, further highlighting the vulnerability of the global benchmark to local supply bottlenecks and squeezes.

A rise in Brent prices is felt by consumers and businesses around the world as Brent is used as a benchmark for pricing around two thirds of the world's oil, including crude produced in Europe, the Middle East, Africa and parts of Asia.

The Brent contract is based on four North Sea crude oils - Brent, Forties, Oseberg and Ekofisk. Export programs for September, due to be revealed next week, are expected to show a sharp drop in output of Forties.

Supply of Forties, the largest stream and the most important for setting prices, is set to fall because Nexen (NXY.TO) plans to close its Buzzard field for maintenance. Buzzard is the largest field of the more than 70 that feed into Forties.

The prospect of lower output is already supporting the price of Brent for immediate delivery compared to later months, analysts say. The spread between the September and October Brent contracts has risen to $1.19, up from 88 cents a week ago. This backwardation could get wider still.

"Declining production in the UK is likely to be a hot subject of discussion during September and October if as we expect the number of Forties cargoes available for benchmarking will fall close to zero," said Olivier Jakob, analyst at Petromatrix.

"The front Brent spread continues to strengthen in anticipation of that and given the squeeze potential on Forties during the maintenance of the Buzzard field we will not propose a limit for the backwardation potential of Brent during September and October."

North Sea oil traders expect the September export programs for Brent, Forties, Oseberg and Ekofisk to emerge early next week, most likely around August 7.

Shipments of the four in August were scheduled to average 774,000 barrels per day (bpd) in August, a record low, hit by maintenance, a strike in Norway and natural declines at the oilfields, many of which are ageing.

Nexen plans to start maintenance work at Buzzard in the first week of September and expects output to be shut down for several weeks before full production returns by mid-October.

A North Sea trader said the full impact on the market would not become apparent until next week, once the loading programs for Forties and the other three benchmark-setting North Sea crudes are issued.

"The expectation is there should be a shorter program for Forties. Even for August it wasn't exactly a full program," the trader said. "Once total availability is known, that's when you will see some influence."

If Buzzard, which is capable of producing 200,000 bpd, is shut completely for the whole of September, the total production shut down would amount to 6 million barrels - equal to ten 600,000-barrel cargoes of Forties.

That would reduce Forties shipments from August's already low schedule of 15 cargoes.

Forties is the most important of the four North Sea crude oils for pricing because, as usually the lowest priced of the four, it sets the price of dated Brent. Dated Brent is used to price around two thirds of the world's oil supply.

Analysts have increasingly said that Brent is vulnerable to distortion since 2011 when a series of production problems mostly at Buzzard - relatively insignificant in global terms - led to increases in global prices.

Surges in Brent for immediate delivery shortly before monthly futures contracts expire became a more common feature of the market last year as the underperformance in the North Sea, as well as the loss of Libyan crude, tightened supply.

The spread between first and second-month Brent jumped to $3.04 in September last year. The surges lessened in the rest of the year as output of Libyan crude, which is mostly priced against Brent, began to recover.