* November WCS last at $20/bbl under WTI
* November synthetic last at $0.25/bbl over WTI
CALGARY, Alberta, Oct 18 (Reuters) - Canadian oil prices fell on Thursday as TransCanada Corp shut its 590,000-barrel-per-day Keystone oil pipeline for three days to investigate a potential defect in the line.
Western Canada Select heavy blend for November delivery last traded at $20 per barrel under the West Texas Intermediate benchmark, compared with a day-earlier settlement price of $17 under the benchmark, according to Shorcan Energy Brokers.
Light synthetic crude for November last traded at a 25-cent per barrel premium to WTI, down from a settlement price on Wednesday of $3 over the benchmark.
The price drop came as TransCanada said it discovered a "small anomaly" in the line during routine maintenance. It shut the line, which carries a quarter of Canada's crude exports to the United States, on Wednesday night as a precaution.
"Once re-start happens, we expect normal operations and flows for the remainder of October," TransCanada spokesman James Millar said. "We may have to make up some volumes in November but we are still evaluating this."
Keystone takes crude from Hardisty in central Alberta to southern Illinois and the storage hub at Cushing, Oklahoma.
Demand for Canadian crude was further weakened on Thursday as BP Plc shut down a 75,000 bpd crude distillation unit at its 337,00 bpd Whiting, Indiana, refinery, according to sources.
The sources had no estimate on how long it would be shut for the current work.