* October WCS trades at $29.25/bbls below WTI
* October synthetic at $6.15/bbl below WTI
CALGARY, Alberta, Sept 16 (Reuters) - Canadian cash crude prices extended losses on Monday, pushed lower by strong supply from the oil sands and seasonal weakness in refinery demand.
Western Canada Select heavy blend for October delivery last traded at $29.25 per barrel below the West Texas Intermediate benchmark, according to Shorcan Energy brokers. That compares with a settlement price of $28.75 per barrel below the benchmark on Friday.
After months of being dogged by delays, Imperial Oil Ltd's output has increased at its Kearl oil sands project in northern Alberta to more than 80,000 barrels per day. The company is targeting the project to reach full capacity of 110,000 bpd by the end of 2014.
"Production is actually showing up", one Calgary-based trader said.
Another crude trader said weaker refining margins driven by lower product prices, especially in asphalt, were contributing to softer demand for heavy crude.
Light synthetic crude from the oil sands for October delivery fell to $6.15 per barrel below WTI, its lowest level since last November. That compares with a settlement price of $4.65 per barrel below the benchmark on Friday.
Synthetic crude prices have fallen sharply from a premium to WTI since a coker at Syncrude's oil sands project came back online in August, boosting supply.
Syncrude is a joint venture of Canadian Oil Sands, Imperial Oil Ltd, Mocal Energy, Murphy Oil Corp , Nexen Inc, Sinopec Corp, and Suncor.
Increased supply outweighed the effect of maintenance at Husky Energy Inc's 82,000 bpd heavy oil upgrader in Lloydminster, Saskatchewan, and maintenance on an upgrader at Suncor Energy Inc's northern Alberta oil sands project, which has cut synthetic crude production by 60,000 bpd.