Friday's explosion at Nexen's Long Lake facility that killed one worker and sent a second to hospital with serious burns was a dark day for the oilsands company, says CEO Fang Zhi.

"Yesterday marks one of the darkest days in Nexen history," Zhi said at a news conference in Calgary Saturday.

"Standing here today to share this kind of information is the worst thing a CEO ever has to do," he said. "But no matter how badly we feel it pales in comparison to the pain that two of our employees' families are experiencing."

Two maintenance workers were refitting the valves on a compressor in the gas compression building in the hydrocracker unit at 3:20 p.m. MT when the explosion occurred, said Ron Bailey, senior vice-president of Nexen's Canadian operations.

Nexen's Long Lake facility is 75 km south of Fort McMurray. (Rick Bremness/CBC) One worker, 52, was killed and a second, 30, was sent to hospital in critical condition. The injured worker is now in the burn unit at the University of Alberta Hospital in Edmonton. Investigators remain on scene.

"Having a fatality or having a serious injury is our worst nightmare," Bailey told reporters. "I cannot imagine the pain and devastation that this has caused for the families and for that we're sorry."

Nexen is investigating the explosion, as is Occupational Health and Safety and the Alberta Energy Regulator.

A stop-work order is in place for the facility located about 75 km south of Fort McMurray, outside the hamlet of Anzac.

Brad Grainger, Wood Buffalo's deputy fire chief, said his crew got the call from Nexen at 4:30 p.m.

"Upon arrival we were put on standby by their fire station," he said. "Our crews actually weren't needed. The Nexen medical people removed the people that were injured and took them into the city, into the hospital.

"They had enough fire crew people on hand so we were just there to assist their needs if they needed them," he said. "As you can imagine, with a scene of this size it does take some time to manage and mitigate."

Second problem in 5 months

The explosion marks the second time the plant has come to the attention of regulators in the last five months.

Last August, Alberta's Energy Regulator suspended 95 pipelines operated by at the Long Lake site after a pipeline owned by the company spilled five million litres of water, sand and bitumen nearby.

Bailey defended the facility's safety record.

"CNOOC (China National Offshore Oil Corp., which owns Nexen) has an extremely high standard of safety," he said.

"All I can say is that we're disappointed and we're working to improve, but we can't speculate on the cause of this accident, whether it had anything to do with our culture or not at this point in time."

The Long Lake facility can upgrade 72,000 barrels of bitumen per day. It began producing crude oil in 2008.