Andrew Cole of Yakima, Washington, on assignment for Exxon Mobil cleans up oil spill along the Yellowstone River in Laurel, Montana, July 6, 2011. Photo: Reuters/John Warner

Salmon, Idaho (Reuters) - Exxon Mobil said on Friday that a pipeline that failed two weeks ago, leaking oil into the Yellowstone River, routinely transported a heavier and more toxic form of crude than the company and federal regulators initially acknowledged.

The Silvertip pipeline carries so-called tar sands crude from Alberta, Canada, as do the U.S. pipelines of most major oil companies, Exxon spokeswoman Karen Matusic told Reuters.

Matusic said the tar sands crude was present along various segments of the pipeline but not at the spill site in Montana.

"Oil from Canada was in the line, but not that area that was affected by the breach. The oil that spilled out, that oil came from Wyoming," she told Reuters, referring to sweet crude produced in oilfields at the Montana-Wyoming border.

Tar sands oil, a synthetic petroleum product derived from tar sands or bitumen, contains more toxic components than the sweet, or low sulfur, crude that Exxon and government regulators initially said flowed in the Silvertip.

Almost all the oil reserves in Canada's Alberta province are found in oil sands, and Alberta accounts for more than 95 percent of Canada's total oil reserves.

A section of the 69-mile long Silvertip pipeline ruptured on July 1 and released what Exxon estimates was 42,000 gallons of oil, or 1,000 barrels, into the Yellowstone River west of Billings.

Officials with the U.S. Transportation Department's Pipeline and Hazardous Materials Safety Administration said late on Wednesday they had just learned that the Silvertip carried oil from Canada.

That a pipeline thought to transport only "sweet" crude could have carried the heavier and more corrosive tar sands crude from Canada has raised concerns by health and environmental officials.

MONTANA GOVERNOR FAULTS EXXON

Tar sands crude may also cause more wear and tear on pipes because of its chemical makeup, including corrosive and abrasive agents, said Tom Finch, the pipeline administration's technical services director for the western regional office.

Federal inspectors were trying to determine if transport of tar sands crude could have triggered internal corrosion that may have played a role in the rupture, Finch said.

Montana Governor Brian Schweitzer has faulted Exxon for failing to tell the state exactly what kinds of crude ran in the pipeline or spell out what hazardous chemicals were in the mix now contaminating riverside properties.

At least five residents have been treated in hospital emergency rooms for dizziness, nausea and respiratory distress they linked to the spill.

"Since they dumped that oil into the river that the state owns and manages, since they have spread oil in a film across 150 separate properties, since the film is over fishing access sites and state parks, we thought it would be appropriate to know what it is," Schweitzer told Reuters on Thursday.

Richard Opper, head of the Montana Department of Environmental Quality, said he was surprised to learn the pipeline buried in the streambed of the Yellowstone may sometimes have moved tar sands crude from Canada.

Exxon has apologized for the spill, which it estimates at 42,000 gallons, and pledged to restore a river prized for its near pristine waters, scenic beauty and abundance of wildlife.

EPA officials are analyzing the chemical fingerprint of the oil which, depending on its source, could contain anything from benzene, a known carcinogen, to hexane, a toxin that can damage the human nervous system.

(Editing by Dan Whitcomb and Peter Bohan)

Source: Reuters