* WCS quoted at $27.50/bbl under WTI
* Synthetic discussed at $1.75-$4.50 under WTI
* Keystone outage, Enbridge apportionment point to glut
CALGARY, Alberta, Oct 25 (Reuters) - Canadian heavy crude prices have tumbled to their lowest in 16 weeks following a five-day outage of the Keystone pipeline, then Enbridge Inc's announcement of double-digit apportionment on much of its U.S. system, trade sources said on Thursday.
Western Canada Select heavy blend for December delivery was quoted at $27.50 a barrel under benchmark West Texas Intermediate, down from $25.50 a barrel under WTI on Wednesday to the lowest since early July.
Light synthetic for December was also assessed weaker in a range of $1.75-$4.50 under WTI though Shorcan Energy Brokers reported no deals in the current period between normal trade windows. Until last week, synthetic had fetched a premium for eight weeks.
TransCanada Corp restarted its 590,000 barrel per day Keystone pipeline to Wood River, Illinois, and Cushing, Oklahoma, from Alberta on Monday following a five-day outage to check on a section in which in-line testing had detected a possible defect.
The company said the longer-than-expected outage would impact deliveries for the rest of this month and that it would work with its customers on ways to make up the losses.
Then on Tuesday, Enbridge announced it would ration space in November on Lines 5, 6A, 62, 64 and 6B, all in the Midwest, as nominations exceeded available capacity.
That, and planned maintenance at some big refineries such as the 306,000 bpd Phillips 66 and Cenovus Energy Inc Wood River plant, point to a glut of supplies within Alberta, market sources said.