By the evening, Syncrude had sent out a tweet saying the fire at the Mildred Lake upgrader had been isolated and was under control.
“All Syncrude personnel and contractors are to report for their night shift as per usual,” said the company.
Alberta Health Services said emergency medical staff transported one person in serious but stable condition from the plant to hospital in nearby Fort McMurray.
The province issued an air quality alert for the Fort McKay area that the fire and smoke could affect health and visibility.
“Take precautions. Stay indoors and keep outdoor activity to a minimum if you have to go outside,” said the alert.
“Keep doors and windows closed and use air purifiers if possible. If you suffer from chronic lung conditions, be sure that you have sufficient medication for at least 2 days.”
Syncrude spokesman Will Gibson said the fire broke out shortly before 2 p.m. and the company’s fire crews began fighting the flames.
He said access to the site had been restricted and workers were leaving the area.
Syncrude employs about 4,600 people in the region. There was no word on how many people were working at the site when the fire broke out.
Melissa Blake, mayor of the Municipality of Wood Buffalo, urged people not to jump to conclusions.
“Anxiety high for those waiting for friends and family to connect,” she said on Twitter. “Don’t let rumours run the social media mill. Facts will come when they can.”
Another fire at the Mildred Lake site in August 2015 cut output from the facility by about 80 per cent. The fire damaged pipes, power and communications lines between two units of the upgrader.
It returned to normal production the following October.