NEW YORK - Cash crude differentials firmed on Wednesday on more favorable spreads as traders began the first day of trading for October barrels.

Mars sour MRS- dealt for as little as $1.70 below West Texas Intermediate CLc1, after September barrels of Mars had dealt for $3.00 below WTI on Tuesday. October Thunder Horse THH- dealt up at 20 cents a barrel above WTI, from September barrels at -25 cents on Tuesday. Light Louisiana Sweet LLS- dealt even with a trade on Tuesday, at +$1.80.

Tuesday marked the end of the three-day roll period and the deadline for September pipeline booking. As trading shifted to October grades Wednesday, the U.S. Department of Energy reported data showing an unexpected, 200,000-barrel build in U.S. crude stocks last week, driven primarily by a 1.1-million-barrel-a-day increase in crude imports. [EIA/S]

U.S. crude futures fell and erased a premium they briefly held over Europe's Brent oil CL-LCO1=R. WTI fell 75 cents after market settlement to $71.30 a barrel, an 11 cent a barrel discount to Brent, which fell 41 cents to $71.41 a barrel.

A Brent premium means that imports, largely priced against the Brent marker, remain less competitive against U.S. grades. WTI for October delivery traded for a 78 cent a barrel discount to November crude, even with Tuesday. A large contango discount tends to strengthen U.S. cash grades.

Among other grades, Bonito sour BS- traded for 10 cents a barrel above WTI. Bonito for September delivery had traded for +$1.65 on August 20. Poseidon sour PSD- traded for -$1.75 a barrel versus WTI, down from -$1.50 on Monday. West Texas Intermediate at Midland WTM- traded for -27 cents. It last traded for September delivery at +30 cents on August 21. West Texas Sour WTS- dealt for -$1.95, after barrels went for -90 cents in roll trade Monday.

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 60 cents a barrel.

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
(Reporting by Joshua Schneyer; Editing by Christian Wiessner)