Cash crude differentials weakened sharply Tuesday on the last day of dealing for September cargoes, as traders scrambled to unload grades.

Mars sour MRS- dealt for $3.00 a barrel below West Texas Intermediate CLc1, down from -$2.00 on Monday. Light Louisiana Sweet LLS- traded for $2.00 a barrel above WTI, down from as much as +$2.90. Heavy Louisiana Sweet HLS- traded at +$2.00, down from as much as +$3.00 a barrel on Monday.

Tuesday marked the end of the three-day roll period and the deadline for September pipeline booking. The roll period is usually marked by erratic prices, with cash traders starting to concentrate on October barrels for the rest of the week.

LLS for October delivery traded for as little as $1.50 a barrel above WTI.

Grade differentials also weakened as U.S. crude futures fell, but traded at a premium over Europe's Brent oil CL-LCO1=R. WTI fell $3.05 after market settlement to $71.32 a barrel, a premium of 7 cents a barrel to Brent, which fell $3.01 to $71.25 a barrel.

A WTI premium means that foreign imports may become competitive against U.S. grades, driving down their differentials. WTI for October delivery traded for a 74-cent discount to November crude, down from a 77-cent discount on Monday. A smaller contango discount tends to weaken U.S. cash grades.

Unexpected refinery maintenance may also have hurt cash grades. Exxon Mobil Corp (XOM.N) said on Tuesday it had unexpectedly shut down a hydrocracker unit at its 348,500 barrel a day refinery in Beaumont, Texas. A spokesman expected minimal impact on production.

Late Tuesday, the American Petroleum Institute reported in weekly data that U.S. crude stocks unexpectedly rose by 4.3 million barrels in the week ending Aug. 21.

Among other grades, Thunder Horse THH- was down $1.25 a barrel at 25 cents below WTI, versus +$1.00 on Monday. The roll from September to October barrels was between -50 and -45 cents, several traders said.

On the West Coast, Alaska North Slope crude oil ASW- for October delivery last sold Monday for 40 cents a barrel over West Texas Intermediate, up 75 cents, traders said.

California buyers lowered posted prices by $2.30 a barrel.

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See for Reuters' generic refining margins

See CL-1=R for the WTI front/second month spread

See CL-LCO1=R for front month WTI/Brent futures spread

See BFO- for Reuters' assessment of Dated Brent

See <0#FRT-> for Reuters assessed tanker rates

See BRNV for assessed domestic crude differentials
See PPGR for outright U.S. cash crude prices

See [REF/US] for a list of U.S. refinery outages

See [EIA/S] for U.S. EIA inventory reports and forecasts

See [CRU/TU] for recent cash crude deals

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(Reporting by Joshua Schneyer; Editing by Walter Bagley)

U.S.