* November WCS last trades at C$16.50/bbl under WTI
* November synthetic last at a $3.35/bbl premium to WTI
CALGARY, Alberta Oct 16 Canadian cash crude prices weakened on Tuesday as refinery work pared demand.
November Western Canada Select heavy grade last sold for $16.50 per barrel under the West Texas Intermediate benchmark, compared with a settlement price of $15.50 per barrel under WTI on Monday, according to Shorcan Energy Brokers.
Light synthetic crude for November last traded at a $3.35 per barrel premium to WTI, down from the Monday settlement price of $4.30 above the benchmark.
Prices have weakened gradually since the start of October when the spreads prompted some shippers to forgo planned shipments to the Cushing, Oklahoma, storage hub as they could not compete with local prices when transport was factored in.
Refinery work is also cutting into demand for Canadian crudes.
Tesoro Corp is conducting planned maintenance at its 120,000 barrel-a-day refinery in Anacortes, Washington, a big buyer of Canadian crude.
Suncor Energy Inc, meanwhile, started planned maintenance at its 93,000 bpd Commerce City, Colorado, refinery this weekend. One of the crude units is slated to be down.
As well, BP Plc is planning to shut the largest crude unit at its Whiting, Indiana, refinery during the fourth quarter as part of a $4 billion upgrade project at the plant. The big crude unit's capacity is 260,000 bpd, according to energy industry intelligence group IIR.