Suncor was shutting down a process unit at its Sarnia refinery Tuesday.
The company said increased noise and flaring is possible.
In a message on the Community Awareness and Emergency Response (CAER) line, spokesperson Jennifer Johnson said the Chemical Valley Emergency Coordinating Organization (CVECO) and Ontario's Spills Action Centre were notified.
She said no community action was required.
Environment Ministry spokesman Gary Wheeler said Suncor reported a leak of crude oil from an above ground pipe. He said the oil was contained on-site and was going to the refinery's waste water plant for treatment.
"The company has monitored the area for odours and volatile organic compounds and has found no off-site odours or detections," said Wheeler in an email to BlackburnNews.com. "The company has also monitored their waste water effluent and there are no elevated readings."
Wheeler said ministry staff are currently inspecting the area and gathering more details from the company, and will continue to monitor air quality using hand-held equipment.
-With files from Josh Boyce