Delaware City residents have reported hearing loud horns and rumbling from the refinery.
One resident called DNREC and was told the fire is ongoing and flaring is happening, according to Facebook.
During flaring, there may be loud rumbling sounds, according to multiple refineries' websites.
Flaring is used to safely burn excess hydrocarbon gases that cannot be recovered or recycled, according to ExxonMobile. Excess gases are combined with steam and/or air, and burnt off in the flare system to produce water vapour and carbon dioxide.
No one at the Delaware City Refinery was injured in a large fire that started about noon Sunday and took almost five hours to get under control, a company representative said.
One firefighter was treated for an unrelated medical issue, the refinery said.
Jamie Bethard, an environmental program manager with the Delaware Department of Natural Resources and Environmental Control, said the agency would be working with the refinery to figure out how the fire started.