Canada crude-Refinery work pressures heavy spreads

* WCS for August quoted at $29.90 under WTI

* Refineries run by Marathon, Imperial among those down

* August synthetic quoted at $5.50 under WTI

CALGARY, Alberta, July 3 (Reuters) - Canadian heavy crude discounts deepened on Tuesday to levels not seen in 3-1/2 months as refinery maintenance in Canada and the U.S. Midwest created a glut of supplies, market sources said.

Western Canada Select heavy blend for August delivery last sold for $29.90 a barrel under benchmark West Texas Intermediate, compared with $27 a barrel under WTI late last week, according to Shorcan Energy Brokers.

WCS discounts were last in the $30-under-WTI range in March.

Among refineries with units out of service for upkeep, Marathon Petroleum Corp said on Tuesday its 206,000 barrel a day plant in Robinson, Illinois, will restart equipment in mid-July following the completion of work.

Imperial Oil Ltd said its 187,000 bpd Strathcona refinery near Edmonton, Alberta, remains in turnaround and work is expected to last to the middle of this month. Parts of its 121,000 bpd Sarnia, Ontario, refinery are also off line and are expected to resume operations by mid- to late July.

Husky Energy Inc had said it plans to take units at its joint-venture Toledo, Ohio, refinery down for 21 days of planned maintenance around now.

Meanwhile, spreads for synthetic crude, derived from the Alberta oil sands, also widened a bit. Light synthetic for August delivery was quoted at $5.50 a barrel under WTI, compared with $5.25 under last week.