The oil, totaling about 65,000 gallons, spilled out of a holding tank at the Caterpillar plant on Channahon Road. Most of it spilled on land.
The Coast Guard has contained the spill in the water, and investigators from the Environmental Protection Agency (EPA) were also at the site Sunday.
Authorities say the spill poses no threat to humans, and a spokesperson with the EPA says that no fish have been hurt.
"The majority of the problem is the land near the facility. The majority of the collection areas that we have, we have not seen any significant collection. We will be continuing to monitor the situation to make sure that the impact is minimal to the fish and wildlife in the area," EPA spokesperson Jim Mitchell told ABC7 Chicago.
Mitchell predicted it would take another two or three days to finish the clean-up process.