The CEO of Nexen has apologized for an explosion at the company’s oilsands facility near Fort McMurray, Alta., that left one worker dead and another in serious condition.

Fang Zhi said at a press conference on Saturday that the explosion, which occurred Friday afternoon at Nexen’s Long Lake facility in Anzac, marked one of the "darkest days" in his company's history.

"But no matter how badly we feel, it pales in comparison to the pain that two of our employees' families are experiencing," said Zhi. "Our thoughts are with the families."

Ron Bailey, the company’s senior vice-president of Canadian operations, also expressed his sadness over the incident.

"Having a fatality, or someone seriously injured, is our worst nightmare," said Bailey. "I cannot imagine the pain and devastation that this has caused for the families, and for that we're sorry."

Bailey said that the two employees worked in maintenance. He added that the seriously injured employee was taken to hospital, and has since been transferred to a burn unit in Edmonton.

Bailey said the employees' names will not be released "out of respect for the families."

In light of the incident, operations at the site are being almost "completely" shut down.

"Obviously, the primary focus right now is the safety and well-being of our people, and we actually need to bring the facilities down in a safe way," said Bailey.

Bailey said the company believes the explosion originated from a compressor building that is part of the facility's hydrocracker unit, which refines gas oil into a "lighter oil product that can be shipped as premium synthetic crude."

He added that the facility was shut down "as quickly as possible" and has been "stabilized."

Bailey said the building was undergoing maintenance at the time of the incident. However, he said that the company does not know what triggered the explosion and has not had full access to the site since the explosion occurred.

"We're going to be investigating the incident and trying to find the root cause," said Bailey.

"I couldn't even speculate today how long that will be," he added.

The company, which is owned by China's state oil company, CNOOC Ltd., had its pipeline licence at the Long Lake facility suspended on Aug. 28.

At the time, the Alberta Energy Regulator said Nexen was not complying with pipeline maintenance and monitoring regulations.

The AER allowed the company to partially resume operations just over a week later.

On Saturday, Bailey said the issues were not related to Friday's explosion.

Bailey added that CNOOC has an "extremely high standard of safety" and will continue to work to improve.

"We can't speculate on the cause of this accident, whether it had anything to do with our culture at this point in time or not," he said.