Trading volumes were very thin however, as the Canadian market is outside the trading "window", which runs from the first of each month until the day before pipeline nominations are due, in which the bulk of activity takes place.

Western Canada Select heavy blend crude for January delivery in Hardisty, Alberta, traded at $28.15 per barrel below the West Texas Intermediate benchmark, according to Shorcan Energy brokers.

WCS also settled at $28.15 per barrel below WTI on Monday, its deepest discount since December 2013.

The hefty discount means the outright price of WCS fell below $30 a barrel, cutting into revenues for Canadian producers and canceling out the benefits of a recent rally in WTI.

Canada's largest pipeline company Enbridge told shippers it will ration more space than usual on its pipelines next month, in the latest sign of high storage inventories in western Canada pushing export networks to capacity.

Inventories hit record highs last week after a nearly two-week shutdown in November of TransCanada Corp's 590,000 barrel-per-day Keystone pipeline because of a leak in rural South Dakota.

Keystone has restarted with a 20 percent pressure restriction, meaning the glut of crude in Alberta will take longer to clear.

Enbridge also announced mid-month apportionment on its Mainline network in December, adding to congestion.

Demand to ship crude by pipeline and rail is outpacing capacity as oil sands production rises, increasing the risk deep discounts on Canadian crude will persist well until at least late 2019.