* LLS sells for $12.70 over WTI, up 70 cents
* Mars trades at premium of $10.70, up 40 cents
* Low outrights, refinery needs drive, not arb
HOUSTON, June 5 (Reuters) - Refiners took advantage of relatively low outright prices to buy oil for July runs, driving U.S. Gulf Coast cash crude differentials higher on Tuesday.
Light Louisiana Sweet LLS- sold for $12.70 a barrel over West Texas Intermediate, up 70 cents from Monday's trades.
Mars sour MRS- traded at a premium of $10.70, up 40 cents from the previous day's session.
Southern Green Canyon sour SGC- and Poseidon sour PSD- bargained even with or stronger than usually superior Mars, and traders cited supply and quality shifts driving the two stronger.
"With futures low, traders waiting on Seaway crude to reach Houston and the WTI-Brent spread still relatively wide, refiners need crude for July," trading consultant John Troland said.
Heavy Louisiana Sweet HLS- sold for a premium of $15.15, about $2.50 stronger than LLS, and traders cited shale crudes weighing on LLS and a comparative shortage of HLS.
"There is limited supply, and HLS is useful to blend with Bakken and other shale lights to get cheaper LLS-like crude," a trader said.
Bakken North Dakota crude BAK- went the other way, selling 15 cents weaker at $11.65 a barrel under WTI. Refinery shutdowns weighed on Canadian and Bakken crudes, traders said.
The transatlantic spread narrowed more than 30 cents but buyer demand prevailed with futures -- and therefore outright physical prices -- down sharply since early May.
On futures markets, July WTI settled 31 cents higher at $84.29 a barrel. July Brent ended regular trading down one cent at $98.84.
(Reporting By Bruce Nichols; Editing by David Gregorio and Jim Marshall)