CALGARY, Alberta Nov 15 (Reuters) - Enbridge Inc said on Thursday it had rationed more oil pipeline capacity on its huge Canada-to-United States system, a rare mid-month move that sent prices for Canadian heavy crude tumbling.
Enbridge said shipper nominations to transport volumes on two pipelines that together can carry nearly 60 percent of Canada's oil exports to the United States will be cut back by 18 percent for the balance of November due to unplanned outages and maintenance.
The company had already rationed space on a number of U.S. Midwest pipelines at the start of the month, after shipper nominations exceeded available capacity, a problem that has plagued producers and refiners as Canadian oil sands and North Dakota unconventional crude output has surged.
The system had already been clogged after TransCanada Corp ran its Keystone pipeline to the U.S. Midwest and Midcontinent area from Alberta at reduced rates in recent days due to power outages in Manitoba.
Enbridge said the new apportionment affects Line 4, which moves up to 796,000 barrels a day to Superior, Wisconsin, from Edmonton, Alberta, and Line 67, which carries 450,000 bpd to Superior from Hardisty, Alberta.
Western Canada Select, a widely quoted heavy crude grade, sank as much as $5 a barrel on Thursday as word of the apportionment spread through the cash market. WCS for December delivery last sold for $32.50 a barrel under benchmark West Texas Intermediate, a $2.50 deeper discount than on Wednesday, according to Shorcan Energy Brokers.