* Maintenance to last for several weeks * June WCS heavy crude discount widens * Nanticoke refinery also undergoing upkeep CALGARY, Alberta, May 8 (Reuters) - Imperial Oil Ltd has taken a processing unit at its Sarnia, Ontario, refinery down for several weeks of unscheduled maintenance following a weekend disruption, a move that pressured Canadian heavy crude oil prices. Imperial said on Tuesday it will manage supply to minimize potential impacts to customers of its 121,000 barrel a day plant. Other parts of the refinery will remain in operation during the work, the company said. "The maintenance work that's currently under way will provide us with an opportunity to conduct regular maintenance projects and also address any issues related to Sunday's operational issue," Imperial spokesman Jon Harding said. On Sunday, a plume of black smoke rose from a coker unit at the refinery, and the company cut crude supply to the equipment, which is used to process heavy oil. The all-clear was five hours after the morning incident, according to the company. On Tuesday, Western Canada Select heavy crude, a frequently quoted Canadian heavy oil benchmark, sold for $16.30 a barrel under West Texas Intermediate for June delivery, a $1.05 deeper discount than on Monday. Traders cited the uncertainty of the Sarnia outage for the drop. Harding declined to give a more precise estimate for the expected duration of the shutdown or the capacity of the coker unit. Imperial is also conducting maintenance on various units at its other southern Ontario refinery, the 112,000 barrel a day Nanticoke plant, Harding said. That work is also expected to last for several weeks.