* Brent trades $8.40/bbl higher than Dubai crude * Brent trades more than $22/bbl higher than U.S. crude * Saudi output hike depresses relative value of OPEC oil (Adds context, Brent's premium to U.S. crude) By Alejandro Barbajosa SINGAPORE, June 14 (Reuters) - Oil output constraints in the North Sea, Nigeria and Libya sent the premium of European benchmark Brent crude over Middle East marker Dubai to its widest level in 6-1/2 years on Tuesday, as Saudi Arabia raises production and cheapens OPEC supplies. The Brent/Dubai Exchange of Futures for Swaps (EFS) jumped 70 cents to $8.40 a barrel by 0430 GMT, Reuters data showed. That is the highest level since December 2004, when a glut of heavy sour crude depressed the relative value of exports from producers including Saudi Arabia, Kuwait, the United Arab Emirates, Iran, Iraq, Qatar, Venezuela and non-OPEC Mexico. Saudi Arabia's solo move to boost output after the Organization of the Petroleum Exporting Countries last week failed to reach a consensus to pump additional supplies is widening the price gap between undersupplied light crude and abundant lower-quality oil. This may force producers to offer their heavy grades to customers at deeper discounts. Lower relative values for high-sulphur crude hurt most OPEC countries and benefit refiners that have the upgrading capacity to process heavy oil to light fuels.

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