* One October cargo, most of September's, said to be delayed
* Brent spread strengthens on expectations of tighter supply
By Alex Lawler
LONDON, Sept 21 (Reuters) - Shipments of North Sea Forties crude oil are likely to be further delayed following lower-than-expected production at Nexen's Buzzard oilfield, trading sources said on Wednesday.
Disruption to Forties shipments has tightened supplies in the North Sea, home of the dated Brent benchmark used to price up to 70 percent of the world's physical crude cargoes, and supported Brent futures prices.
Forties operator BP Plc was looking this week for cargo owners to volunteer to have their shipments delayed, one of the sources said. The operator has not required cargo owners to take delivery later.
"There have been opportunities to defer but there's no sign of anything being forced," said one source, whose company is an equity producer of Forties cargoes.
"I have not seen confirmed deferrals yet but they must be on the way," said another trading source.
One of October's 23 Forties cargoes has already been deferred -- Shell's cargo originally due to load on Oct. 4-6 -- as have most of September's shipments. Each cargo is of 600,000 barrels.
Supply of Forties is important for world markets as it usually sets the value of dated Brent.
The prospect of more deferrals follows a drop in production at Buzzard, which normally pumps about 200,000 barrels per day (bpd) and has yet to return to planned volumes, the sources said.
"Buzzard is playing up again. It's still below plan," said the source at the equity producer of Forties. He said output has been as low as 50,000 bpd and as high as 160,000 bpd in the last few days.
Nexen on Tuesday did not respond to a request for comment on output at the field. Last week, the company said production was varying day to day as it completed the tie-in of a fourth production platform.
Forties shipments have been subject to an unusual level of delays and cancellations since May partly due to problems at Buzzard, the UK's largest oilfield and normally the biggest contributor to Forties output.
Reduced North Sea output has helped drive up Brent-related physical and futures prices. Forties' premium to dated Brent is at a record high.
November Brent futures are trading $1.42 a barrel above December, around 20 cents more than on Tuesday, reflecting relatively tight supplies in the near term.
"The spreads in Brent strengthened again since there is no immediate cure to the Buzzard syndrome," Olivier Jakob, analyst at Petromatrix, said in a report.
