Oct 10 (Reuters) - Exxon Mobil Corp said on Monday it had no reason to believe an oil sheen on the Yellowstone River in Montana came from its Billings refinery but it had moved to contain the oil to minimize any environmental impact. "At approximately 3:30 p.m. on Friday, October 7, an oil sheen was observed on the Yellowstone River upstream of the refinery," Exxon spokeswoman Rachael Moore said. Moore said an oil-spill response team for the 60,000-barrels-per-day refinery had laid a containment boom at the point of origin. A spokesman for the Montana Department of Environmental Quality said ConocoPhillips' 58,000-bpd refinery in Billings is located north of Exxon's on the river and that they shared some pipelines and infrastructure. A spokesman for ConocoPhillips said it did not believe the sheen was related to them. "We are aware of this report and conducted our own investigation; we found no reason to believe it is related to our operations," said Rich Johnson, the ConocoPhillips spokesman. Exxon said Billings refinery operations are normal.