* June WCS quoted at $17.50/bbl under WTI * Unit at Imperial's Sarnia plant down for several weeks * Light synthetic quoted at $2.25 over WTI CALGARY, Alberta, May 8 (Reuters) - Canadian heavy crude prices weakened on Tuesday as an unscheduled outage of a unit at Imperial Oil Ltd's Sarnia, Ontario, refinery led to a back-up of supplies in the U.S. upper Midwest, market sources said. Western Canada Select heavy blend for June delivery last sold for $17.50 a barrel under benchmark West Texas Intermediate, a $2.25 deeper discount than on Monday. Imperial said on Tuesday that a coker unit at its 121,000 bpd Sarnia plant will be down for several weeks of maintenance following a weekend disruption, and that the shutdown would allow it to conduct other work at the plant as well. Other parts of the refinery will remain operating during the work, the company said. The outage has prompted players to seek delays to supplies on the Enbridge pipeline system into Stockbridge, Michigan, and Sarnia, an oil marketing source said. "(There is) a bit of a glut forming," the source said. Imperial said parts of its 112,000 bpd Nanticoke, Ontario, refinery are also down for several weeks of planned work. Potentially affecting business on Wednesday, it was reported that an incident at the 74,000 bpd Sinclair Oil refinery in Wyoming sent four workers to hospital. Local media said the incident was at the gas recovery unit. Impact on the plant's operations was not immediately known. The refinery gets Canadian crude from Kinder Morgan's Express pipeline. Light synthetic for June also weakened on Tuesday. It last sold for $2.25 a barrel over WTI, compared with a $2.50 a barrel premium on Monday.