(Reuters) - Exxon Mobil Corp said on Tuesday that the release of an unidentified substance from a vessel into the Des Plaines river in Illinois on Monday was minor and not related to its Joliet refinery operations.
The company said Joliet refinery personnel discovered a petroleum sheen on the river adjacent to the refinery and cleaned it up.
"The source is not believed to be related to refinery operations," company spokeswoman Tricia Simpson said in an email on Tuesday.
Exxon runs the 238,600-barrel-per-day Joliet refinery in Channahon, Illinois.
(Reporting by Koustav Samanta in Bengaluru; Editing by Jeffrey Benkoe)