(Adds details on injured worker, comment from provincial health and safety official)
By Jeffrey Hodgson
TORONTO, Jan 16 (Reuters) - Nexen Energy executives apologized on Saturday for an explosion at the company's Long Lake oil sands facility in Alberta that killed one employee, left another critically injured and shut down the site for an indefinite period.
Fang Zhi, who heads the unit of China's CNOOC Ltd, said the incident marked one of the darkest days in Nexen's history.
"Our thoughts are with the families," he told reporters in Calgary.
The company said the injured worker had been taken to the burn unit of a hospital in the provincial capital of Edmonton.
The explosion happened on Friday afternoon at the site south of Fort McMurray, Alberta.
Ron Bailey, Nexen's senior vice-president of Canadian operations, said the facility was shut down as quickly as possible.
The company has also shut down its upgrader there and stopped its steam-assisted gravity drainage operation, resulting in a complete halt to production.
The company is working with regulators and conducting its own investigation. Bailey said there was no schedule for resuming production at the site.
"The priority right now is to shut down safely, suspend operations and understand what went on," he said. "I couldn't even speculate today as to how long that will be."
The lost production will be just under 50,000 barrels a day, he said.
Bailey confirmed that the explosion happened in the facility's hydrocracker unit.
According to the company's website, the hydrocracker is where hydrogen is combined with partially upgraded oil to remove sulfur and produce synthetic crude.
A spokeswoman with the Alberta government's Occupational Health and Safety arm said the explosion happened as the workers were changing out valves on a compressor.
She said a "stop work" order was in place for the facility until it is deemed safe to start operating again but could not speculate on how long that would take.
The incident comes after Nexen discovered a pipeline leak last July near the same facility that caused one of North America's largest oil-related spills on land. It apologized soon afterward for that leak.
Last August, the Alberta Energy Regulator ordered Nexen to shut 95 pipelines at the Long Lake facility as part of an investigation into the spill. It resumed full production in September. (Editing by Lisa Von Ahn)