A spokeswoman for the Exxon Mobil refinery in Billings says workers are cleaning up a small oil sheen found in a side channel of the Yellowstone River in Billings.

Exxon Mobil spokeswoman Pam Malek said Monday that booms are in place to contain the oil. She says the oil is not believed to have come from the refinery but the source remains under investigation.

Malek described the sheen as "very small" and says it was discovered by the company Friday on a stagnant channel adjacent to Exxon Mobil property but upstream of the refinery itself.

The Environmental Protection Agency has been notified of the sheen. A spokesman for the federal agency says the amount of oil in the water was not enough to be a measurable quantity.

Associated Press