North Sea oil output is expected to dip 0.6% in September from an already tight August as seasonal maintenance restricts supply, according to a report.

Output from 11 of the main British and Norwegian crude streams will average 1.768 million barrels per day in September, based on the latest information from loading schedules and trading sources, Reuters reported.

The restricted supply outlook is likely to keep North Sea crude oil differentials elevated, while the September Brent futures contract is already trading at a $1.28 a barrel premium to October, reflecting the limited availability, according to data tracked by the news wire.

Although Forties volumes are expected to bounce back following a tight August programme, maintenance in the Norwegian sector will impact crudes such as Oseberg and Asgard, which face a big drop off in September.

The loading programme for Flotta, the smallest stream tracked by Reuters, has not yet emerged.

Some 1.798 million bpd will load from 12 streams in August, which would mean a month-on-month fall of 1.7% if there is no Flotta cargo in September.

Analysts and traders said the programmes looked particularly tight at the start of the month, and said ongoing maintenance might make the volumes difficult to hit.

“There is also a fear of unplanned outages that could add to choppy trading as the fields are so old,” said Andrey Kryuchenkov, an oil analyst at VTB Capital. “Either way, the downside (for Brent) is limited.”

August’s Forties programme has been disrupted due to a slow ramp up by Britain’s biggest oilfield Buzzard following planned maintenance on the Forties pipeline. A second shutdown by the pipeline late last week meant Buzzard had to suspend production.

More than half the August programme is now delayed, and the last cargo on the programme has been pushed into September. Traders are worried that more cargoes could be deferred into September as well as the possibility that some will be dropped entirely.

Brent futures and North Sea crude differentials are also supported by disruptions to Libyan exports, tighter Russian Urals and Azeri supplies, and force majeures for Nigeria’s Bonny Light and Brass River crudes.