Nexen also released the cause of a 2015 pipeline leak that dumped five million litress of bitumen and water into the muskeg
The company idled an upgrader at the Long Lake site after an explosion in January killed two people.
After a review, Nexen has concluded that a short-term repair for the upgrader is not feasible and therefore it is putting it into winter preservation mode with no date set for a restart.
Nexen says the explosion was the result of work being performed outside the scope of approved work activities.
The company has also finished an investigation into a pipeline leak last year that spilled about five million litres of bitumen, sand and produced water over a 16,000-square metre area.
The company concluded that the rupture happened on June 11—about a month before it was discovered by a contractor—was a result of an incompatible pipeline design for the muskeg ground conditions.