Imperial says there was no fire at the Sarnia manufacturing site Thursday night, just flaring as a result of an operational problem.
Spokesman Jon Harding concedes it was very visible with three of four flares on site lit up due to a "mechanical issue of some kind."
Photos and video taken from across the St. Clair River in Michigan gave the impression of an inferno at the plant.
Harding says there were no injuries in the event that began around 6:20pm. The all clear was issued around 8:30pm.
Sarnia Fire and Rescue Platoon Chief Bill Sheane says they also responded to help extinguish a small grass fire around a stack on the fence line between Imperial and ARLANXEO.
Harding says efforts were continuing to safely return the plant to normal operations and flaring was still visible Friday. The company was advising the public that the plume from the site's main boiler stack may be darker than normal at times as the coker unit is restarted.
The Ministry of the Environment and Climate Change says the Imperial plant lost "air supply" to several processing units, which led to their shutdown.
Gary Wheeler says "the facility used flaring as a safe and controlled way to burn off materials left over in the processing units." He says both the company and an independent consultant conducted air monitoring and no abnormal contaminant levels in the air have been identified.
Wheeler says ministry staff are continuing to monitor the situation and will work with the company to gather additional information about the cause of the flaring and whether there is any correlation between the flare operation and the grass fire.