It was not clear what the issue was, but four trading sources said the plant appeared to have been running low for the last two weeks.

WCS for February traded last on Wednesday at $13.70 a barrel below U.S. crude. A week ago, it traded at a $15.35 a barrel discount.

Total net generation for Kearl (IOR3)'s cogeneration unit in Alberta was down to 10 megawatts from a maximum capacity of 84 MW by Thursday morning, according to the Alberta Electric System Operator's website.

(Reporting by Catherine Ngai and Liz Hampton; Editing by Bernadette Baum)