Canada crude-Synthetic, heavy grades weaken

* Nov synthetic quoted at $9.75/bbl over WTI

* Smallest premium since Sept 4.

* WCS quoted at $11/bbl under WTI

CALGARY, Alberta, Oct 3 (Reuters) - Canadian cash crude prices weakened on Wednesday on a wave of selling as North American benchmark prices tumbled and refinery outages increased.

Light synthetic crude for November delivery last sold for $9.75 a barrel over benchmark West Texas Intermediate, compared with $12 a barrel over WTI on Tuesday, according to Shorcan Energy Brokers. That was its smallest premium in a month.

November Western Canada Select heavy blend was quoted at $11 a barrel under WTI, an 80-cent wider discount than on Tuesday.

"Sellers finally woke up," a marketer said.

Canadian crude prices have been strong this month, leaving little or no room for profit in the Cushing, Oklahoma, market when transport costs are factored in, traders have said.

Enbridge Inc said last week it had no need to ration space on its Spearhead pipeline to Cushing from Illinois next month as nominations fell. It was the first month with no apportionment on that line since January.

But in recent days, some refineries have taken units down for repairs, pointing to weaker demand. The 362,000 barrel Wood River, Illinois, refinery run by Phillips 66 and Cenovus Energy Inc , began planned maintenance last week.

On Tuesday, local media reported Exxon Mobil Corp's 60,000 bpd Billings, Montana, plant began planned work to replace part of a fluid catalytic cracking unit.

BP Plc reported maintenance on a flare gas recovery compressor at its 225,000 bpd Cherry Point, Washington, refinery, according to a filing with regulators.

The cash differentials weakened as benchmark prices tumbled in response to disappointing economic data from China and Europe, which reinforced concerns about slowing growth and weak petroleum demand. Front-month WTI was down 3.6 percent at $88.60 a barrel on Wednesday afternoon.

(Reporting by Jeffrey Jones)

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