Planned maintenance is underway at Suncor's Sarnia refinery.
The scheduled turnaround is expected to take about seven weeks to complete.
The company said every effort is being made to ensure disruptions from the maintenance work are kept to a minimum.
About 1,200 employees and contractors will be involved in the project.
Last Thursday, Suncor began the process of shutting down units in preparation for the turnaround. Residents may notice increased flaring and periods of noise as a result.