* April WCS quoted at $23 under WTI * April synthetic $5.50/bbl over WTI * Saskatchewan refinery plans 6-week turnaround CALGARY, Alberta, March 6 (Reuters) - Canadian heavy crude spreads tightened on Wednesday as the market weighed Imperial Oil Ltd's delay in starting up its Kearl oil sands project in Alberta, trade sources said. Western Canada Select heavy blend for April delivery last sold for $23 a barrel under benchmark West Texas Intermediate, the smallest discount in about three weeks, according to Shorcan Energy Brokers. That compares with a Tuesday settlement of $24.75 under WTI. Traders have been surprised by the strength in WCS heavy as major fundamentals have changed little since discounts in January that topped $40 a barrel at times. Influencing factors include tight pipeline capacity and growing industry-wide production, in addition to maintenance at some large Canadian and U.S. refineries. However, Imperial has delayed the start-up of the 110,000 barrel a day Kearl project from an initial target of the late 2012, keeping large expected volumes off the market. Most recently, the company has said it expects commercial production before the end of the month, though the ramp-up is projected to be gradual. April light synthetic crude was quoted at $5.50 a barrel over WTI, down $1 from Tuesday. In refinery news, Federated Co-operatives said it expects to start a six-week maintenance outage at its 145,000 barrel a day Regina, Saskatchewan, refining complex at the end of this month.