UPDATE 1-Syncrude's output forecast cut by biggest owner

* Production target lowered 4.6 pct

* Second guidance change in two months

* Says planned upgrader maintenance extended

CALGARY, Alberta, Sept 23 (Reuters) - Canadian Oil Sands Trust COS_u.TO, the biggest stakeholder in the Syncrude Canada Ltd joint venture, lowered its target for the project's production this year by almost 5 percent as work on an upgrader extends longer than expected.

The trust said it now estimates Syncrude's annual production to be about 105 million barrels, down from its July estimate of 110 million and 8.7 percent below its original 2010 estimate of 115 million barrels.

The trust, which owns a 36.7 percent share of Syncrude, said it was lowering its guidance because of some unplanned outages and because it expects that planned maintenance now under way at one of its upgraders -- which converts bitumen stripped from the oil sands into refinery-ready synthetic crude -- will extend beyond the 45 days it initially forecast.

It did not say when it expects the work to be completed and no one from the trust could be immediately reached for comment.

The project's other owners include Imperial Oil Ltd (IMO.TO), with 25 percent; Suncor Energy Inc (SU.TO), with 12 percent; Sinopec Corp (600028.SS), with 9 percent; Nexen Inc (NXY.TO), with 7.2 percent; while JX Holdings Inc (5020.T) unit Mocal Energy and Murphy Oil Co (MUR.N) each have 5 percent. (Reporting by Scott Haggett; editing by Rob Wilson)