Canada crude-Prices weaken as output set to rise

* November synthetic quoted at $4.50/bbl over WTI

* WCS quoted at $15.20/bbl under WTI

* Suncor maintenance due to end this month

CALGARY, Alberta, Oct 5 (Reuters) - Canadian cash crude prices weakened for the fourth straight session on Friday as production from the Alberta oil sands looked set to increase for next month.

Light synthetic crude for November delivery was last quoted at $4.50 a barrel over benchmark West Texas Intermediate, down from $7.25 a barrel over WTI on Thursday, according to Shorcan Energy Brokers. The premium has shrunken by $7.50 since Tuesday.

Western Canada Select heavy blend for November fetched $15.20 a barrel under WTI, compared with $14.20 under a day earlier.

Supply constraints had been one factor pushing up Canadian crude prices for the past month and a half, a trade source said. Suncor Energy Inc is in the midst of a six-week turnaround on a vacuum tower at an upgrader at its northern Alberta oil sands operation.

That cut production to 300,000 barrels a day in September from 373,000 the month before, though the work is scheduled to be finished in the coming weeks.

Meanwhile, weather-related problems cut production at Syncrude Canada Ltd, to 318,900 bpd from 359,000 bpd.

Some refineries have also started maintenance recently, cutting into demand. The 362,000 barrel Wood River, Illinois, refinery run by Phillips 66 and Cenovus Energy Inc , began planned work last week.

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

(Reporting by Jeffrey Jones; Editing by M.D. Golan)

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