Canada crude - Heavy grades soft as pipeline rationing announced

* Dec WCS trades at $36.00/bbl below WTI

* Dec synthetic trades at $13.00/bbl below WTI

* Enbridge (Toronto: ENB.TO - news) announces December apportionment

CALGARY, Alberta, Nov 21 (Reuters) - Canadian heavy crude prices held near multi-month lows on Thursday, with some traders citing concerns about increased congestion in one of Enbridge (Frankfurt: 885427 - news) Inc's main export pipelines to the United States.

Western Canada Select heavy blend for December delivery last traded at $36.00 per barrel below the West Texas Intermediate benchmark, according to Shorcan Energy brokers, unchanged from Wednesday's settlement price.

Trading volumes were thin as the Canadian crude market is outside the nearly three-week-long trading "window" - between the first of the month and the day before pipeline nominations are due - when most trading takes place.

Prices have recovered slightly since hitting a 10-month low of $41.50 per barrel below WTI on Nov. 5, but market players in Calgary said concerns about limited space on pipelines and crude getting bottlenecked in Alberta meant gains were likely to be limited.

Enbridge, whose pipelines carry the bulk of Canadian crude oil exports to the United States, said it will ration space on five lines in December.

Although apportionment on four lines was reduced from November, it rose on the 231,000 barrel per day Line 6B, which runs between Griffith, Indiana, and Sarnia, Ontario.

Line 6B will be apportioned by 45 percent in December, meaning producers will only be able to ship 55 percent of nominated volumes.

"It's a big number on a line that mostly carries heavy crude. It's certainly a problem for the industry as a whole. It just throws a wrench into everything," said one Calgary crude trader.

Light (Other OTC: LGSXY - news) synthetic crude from the oil sands for December delivery strengthened slightly to trade at $13.00 per barrel below WTI. On Wednesday, synthetic crude settled at $14.00 per barrel below the benchmark.

Royal Dutch Shell Plc (Xetra: R6C1.DE - news) reported minor routine maintenance at its 100,000 bpd Scotford refinery near Edmonton, Alberta, but said there would be no material impact on production.

Meanwhile, Imperial Oil Ltd (Toronto: IMO.TO - news) said operations were normal at its 187,000 bpd Strathcona refinery, also near Edmonton, after flaring was reported.