* April synthetic quoted at $21/bbl under WTI
* April WCS quoted at $34/bbl under WTI
* Syncrude adds 4.7 mln bbls to April outlook: sources
CALGARY, Alberta, March 13 (Reuters) - Canadian synthetic crude prices tumbled on Tuesday on word that Syncrude Canada had revised its April output forecast upward by 4.7 million barrels after pushing back planned maintenance on a major processing unit.
Light synthetic crude for April last sold for $21 a barrel under benchmark West Texas Intermediate oil and widened to as much as $21.50 under. That compares with $16.75 under WTI early in the session.
"There are just going to be a lot more barrels on the market in April," a trader said.
Market sources said Syncrude, one of the two largest Canadian oil sands operations, raised April output by 4.7 million barrels over its previous outlook, extra volume that averages 157,000 barrels a day.
March output, meanwhile, has been cut by 2 million barrels from the last projection, the sources said.
Siren Fisekci, spokeswoman for Canadian Oil Sands Ltd , Syncrude's largest interest owner, declined to comment on monthly production forecasts.
Last week, Syncrude idled its Coker 8-1 for 30 days after determining extra work would be required following a small fire on the unit early this month.
That prompted it to push back a 45-day turnaround of Coker 8-3 beyond the previously scheduled April time frame.
The units turn bitumen from the Alberta oil sands into refinery-ready light crude. Syncrude's overall capacity is 350,000 barrels a day.
Canadian Natural Resources Ltd's 110,000 barrel a day Horizon oil sands project, off line since early February, is also expected to be back in operation by April.
At $21 under WTI, synthetic is close to the record wide spreads of early February, blamed on rising production and limited pipeline capacity out of Western Canada.
Meanwhile, Western Canada Select heavy blend for April was quoted at $34 a barrel under WTI, flat with Monday's differential.