The differential was as narrow as $15.25, the smallest in nearly one month, before the Oct. 29 Keystone pipeline shutdown.
Buying was related to filling nominations for movement on the Enbridge Inc Mainline, and that buying was delayed amid earlier uncertainty about how long the Keystone outage might last, a trader said.
A second industry source said that the narrower differential makes sense factoring in current prices at Houston for heavy oil, and rail costs.
Thursday was the last day of the monthly trading cycle for December deliveries.
TC Energy Corp told a majority of Keystone oil pipeline shippers that November volumes would be cut by nearly 39% after a leak in North Dakota spilled more than 9,000 barrels about two weeks ago, sources familiar with the matter said Wednesday.
Light synthetic crude from the oil sands traded at $2 below WTI, a smaller discount than Wednesday’s settle of $2.15 under.
Oil prices were mixed as U.S. crude futures were pressured by a build in domestic inventories and record production, while forecasts from the Organization of the Petroleum Exporting Countries for a lower-than-expected oil surplus supported Brent.
The Canadian province of Alberta has in recent weeks loosened rules on mandatory oil curtailment by allowing production above set levels if it moves by rail, and by exempting new conventional wells from the restrictions.