UPDATE 1-Platts unveils changes to bolster Brent oil benchmark

LONDON, March 15 (Reuters) - Oil pricing agency Platts on Friday announced changes to the way it assesses the Brent oil market, in a bid to bolster the credibility of the benchmark used to set oil prices globally.

At stake is the credibility of Brent, which sets the price of billions of dollars of daily oil trade. The benchmark is based on a dwindling supply of four North Sea crudes, which critics say leads to a smaller market that is prone to manipulation and can lead to higher global prices.

Platts said it plans to apply quality premiums for Oseberg and Ekofisk crude from June 2013 - two of the four crudes deliverable into Brent-Forties-Oseberg-Ekofisk (BFOE) forward contracts that help establish the price of dated Brent, used to price oil around the world.

"The introduction of quality premiums offers an incentive for more deliveries of Oseberg and Ekofisk and thus has the potential to further gird the supply of oil underpinning the BFOE complex and the dated Brent price assessment," Dave Ernsberger, Platts global editorial director, oil, said in a statement.

The plan from Platts, a unit of McGraw-Hill, follows feedback from oil companies on proposals announced on Feb. 18, which differed from new trading terms earlier detailed by Royal Dutch Shell and backed by BP Plc.

The rival proposals aroused concern among analysts and commentators about a split in liquidity between a market based on Platts and one backed by some oil companies, which could damage the credibility of the benchmark.

Platts also said on Friday the quality premiums will reflect 50 percent of net price differences and will draw on two months of price assessment data. The company had initially proposed using one month's data.