UPDATE 1-Syncrude Canada output cut after unit outage

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CALGARY, Alberta, Oct 1 (Reuters) - A key processing unit at the Syncrude Canada Ltd. oil sands operation in Alberta has gone off line, cutting output at the sprawling facility by 20 percent, its biggest owner said on Monday.

Canadian Oil Sands Trust (COS_u.TO), which has a 37 percent interest in the oil sands mining venture, said coker 8-3 suffered an operational upset on Sunday, reducing output by 70,000 barrels of synthetic crude a day.

Repairs to the unit, part of the plant that turns heavy crude wrung from the oil sands into refinery-ready light oil, could take a few days, the trust said.

But if an investigation reveals the need to remove coke deposits, the coker could stay down for three weeks, it said.

Syncrude has started to boost production from its other two coking units to help make up for the shortfall.

The trust said it could not immediately estimate the impact of the shutdown on its October production or cost targets. The operation's normal capacity is about 350,000 barrels a day.

Canadian Oil Sands trust units were down 40 Canadian cents to C$32.60 on the Toronto Stock Exchange on Monday.

Syncrude, located north of Fort McMurray, Alberta, is the world's largest producer of synthetic crude oil.