* Forties weakens 30 cts to to dated minus 55 cts
* Swaps backwardation flattens at front end
* Prompt physical over-supplied as W. African competes
* But market likely to rise next month as Buzzard shuts
LONDON, July 25 (Reuters) - North Sea crude oil differentials slipped further on Wednesday, undermined by weak demand for prompt cargoes from local refiners and competition from incoming good quality West African crudes.
Jetty maintenance at Forties loading terminal Hound Point also helped pressure the prompt market as it is preventing VLCC loadings for destinations further afield, including exports to South Korea, which has bought two cargoes per month this year.
But traders said the weakness was expected to be relatively short-lived because the Buzzard oilfield, Britain's largest, is due to close for scheduled maintenance in September.
"I think we have found a floor here," said one trader. "Mercuria came in and bought a cargo yesterday and if guys like that start buying, it usually means they see good value."
Traders said cargoes were trading well further out. With the Buzzard field offline, the overall quality of Forties will improve and there will be less Forties around, he said.
"(I would say) two weeks of weakness and then things should take off," one trader said.
The September/October spreads remained well bid, with market participants expecting the price of BFOE to be set by either a higher quality Forties, or Brent.
* Shell sold an Aug. 7-9 Forties cargo to Gunvor at dated Brent minus 55 cents, down from a deal on Tuesday at dated Brent minus 25 cents.
* This was the lowest level for Forties since July 2, according to Reuters data.
* BP offered an Aug. 10-12 loading Ekofisk cargo at dated plus $1.20, unchanged from Tuesday's offer.
* The backwardation at the front end of the curve flattened, as the prompt physical market displayed signs of oversupply.
30-03/8 Oct +85
6-10/8 Oct +75
13-17/8 Oct +79
20-24/8 Oct +84
28-31/8 Oct +79
3-7/9 Oct +59
For a database of oil supply and demand fundamentals upstream and downstream, Reuters subscribers can click here (Reporting by Claire Milhench; editing by Christopher Johnson)