When ExxonMobil's Pegasus pipeline ruptured last month in Mayflower, Ark., it was carrying diluted bitumen, a controversial form of oil from Canada's tar sands region. That was confirmed in a letter an Exxon lawyer wrote to the U.S. Environmental Protection Agency last week.

But the letter contradicts public assertions by company officials that the spilled oil was simply "heavy oil," not tar sands bitumen. It also raises, once again, the question that surfaces after every spill involving oil from Canada: Is it or isn't it diluted bitumen?

Bitumen is a semi-solid form of crude oil found in Canada's vast oil sands region, where it is found with sand, clay and water in formations dating back hundreds of millions of years. Because bitumen is so thick and tarry, producers dilute it with natural gas liquids or light oil so it can flow through pipelines. That creates a type of oil called diluted bitumen, or dilbit.

The letter Exxon sent to the EPA on April 10 was a response to the EPA's request for more information about Wabasca Heavy—the oil that poured out of a 22-foot-long gash on its Pegasus line on March 29. "Can the oil accurately be described as tar sands oil, or a type of diluted bitumen (dilbit)?" the EPA had asked in an April 5 letter to Exxon.

The answer is yes, according to oil producers' definitions of what counts as dilbit, Exxon said. "Canadian producers report their production of Wabasca Heavy as bitumen," wrote Richard Byrne, Exxon's assistant chief attorney of environmental and safety law.

Byrne went on to say that Exxon buys its Wabasca Heavy from Canadian Natural Resources Limited and Cenovus Energy Inc.—two of Canada's biggest oil sands producers. Both companies dilute the bitumen to make it thin enough to meet pipeline specifications, the lawyer wrote.

Other industry groups also describe Wabasca Heavy as dilbit.

A recent report commissioned by the Canadian Energy Pipeline Association, a trade organization, included Wabasca Heavy in a group of eight dilbit samples. And the Battelle Memorial Institute, a private science and technology company in Ohio, counted Wabasca Heavy among seven dilbit samples in a July 2012 study on pipeline corrosion commissioned by the American Petroleum Institute.

Exxon spokesman Alan Jeffers spoke with InsideClimate News about the discrepancy between Exxon's definition of Wabasca Heavy and the definition offered by the Canadian oil producers.

Jeffers said the Alberta government's Energy Resources Conservation Board, an independent agency, told Exxon "that they consider anything from that area bitumen." By contrast, "we consider [Wabasca] heavy crude," not dilbit.

He chalked up the difference to the "varying definitions" and "colloquialisms" associated with the term dilbit.

Jeffers did acknowledge that Wabasca Heavy has similar chemical properties to other types of dilbit, which is crucial when assessing public health dangers and cleanup techniques. "Of course it's going to have many of the same characteristics, because it's from the same geological time and formation," he said.

But Jeffers also said that when pure Wabasca Heavy crude oil comes out of the ground, it's lighter than "what is traditionally known as bitumen."

In its letter to EPA, Exxon submitted a Material Safety Data Sheet prepared by Cenovus, which said that in terms of chemical composition and density, Wabasca Heavy is the same as four dilbits and two non-dilbit heavy oils. Its list includes Cold Lake, the type of dilbit that spilled out of ruptured Enbridge Inc. pipeline in Marshall, Mich. in 2010. After that spill, the diluents began evaporating and the bitumen sank to the bottom of the Kalamazoo River, where it is still being removed today, almost three years later.

Like the other dilbits, Wabasca Heavy contains bitumen blended with a hydrocarbon diluent, usually natural gas liquids; benzene, a known human carcinogen; and hydrogen sulfide, a corrosive and poisonous chemical compound, according to the Cenovus MSDS. Wabasca Heavy also contains at least eight other hazardous constituents, including N-hexane and naphthalene, according to a separate data sheet that Exxon provided to cleanup workers in Arkansas.

Between 210,000 and 294,000 gallons of Wabasca Heavy crude oil spewed out of the 65-year-old Pegasus pipeline nearly three weeks ago, forcing the evacuation of 22 homes in a Mayflower subdivision.

Anthony Swift, an attorney at the Natural Resources Defense Council (NRDC) who studies pipeline safety issues and who wrote a blog post in the wake of the Arkansas spill called "The Tar Sands Name Game," said he believes Exxon and other industry players are trying to confuse public opinion on Wabasca Heavy. The goal, he said, is to avoid bad publicity about tar sands oil at a time when the industry is trying to dramatically increase the amount of dilbit brought into the United States through expanded, repurposed and new pipelines, including the proposed Keystone XL.

"The oil industry is trying to protect the brand of tar sands [crude], and it's happening at the expense of the public interest," Swift said.

Exxon did not return a request for comment on Swift's statement by deadline.

The Keystone XL would carry up to 830,000 barrels a day of Canadian dilbit to Gulf Coast refineries. The Canadian government and oil industry are lobbying hard for the Obama administration's approval of the project, while environmental groups want the pipeline blocked or delayed because of climate change and safety concerns. Those concerns have been exacerbated by Exxon's spill in Arkansas.

Exxon operates one of the biggest oil sands operations through its majority-owned subsidiary Imperial Oil Ltd., one of Canada's largest petroleum companies. Imperial Oil produces more than 200,000 barrels of tar sands bitumen per day, roughly 13 percent of the 1.5 million barrels produced across Canada.

Conventionally Produced. So What?

Wabasca Heavy is produced in the Wabasca oil field of the oil sands region in northern Alberta, the smallest of four oil sands deposits in Alberta. It often appears on maps as part of the Athabasca deposit.

Exxon says one reason Wabasca Heavy isn't bitumen is because of how it's produced. Unlike most bitumens, Wabasca Heavy is extracted using "secondary" recovery methods, the same techniques that are used to drill for heavy crudes in Saudi Arabia and other major oil deposits.

While primary conventional crude oil production involves drilling a well so that the oil flows up largely by itself from the pressure, in secondary production, the oil needs more help getting out of the well—either because it's too thick or too difficult to reach. Developers can drill horizontal wells and inject water, solvents, or a mixture of non-toxic polymers in water to force the oil out of the ground. These are considered conventional "enhanced oil recovery" techniques, and this is how Wabasca Heavy is produced, according to industry experts who spoke with InsideClimate News.

Other dilbits, including Cold Lake blend, are produced unconventionally—either by mining chunks of bitumen and separating them from sand and water, or by injecting steam into the ground to melt the crude oil.

But oil produced using conventional methods is still considered bitumen, according to Canada's National Energy Board, a national regulatory agency. Conventional oil production is an "important component of the total oil sands picture," a 2006 report by the agency said, accounting for nearly 10 percent of total bitumen production at the time.

According to Bill Lywood, president of Alberta-based Crude Quality Inc., an industry consultant, heavy oil, extra-heavy oil and natural bitumen are found together in formations throughout the tar sands region. "Depending on how you produce it, you will get varying amounts of those three materials," he said.

March 30 screenshot of Wabasca Heavy listed as dilbit/Source: CrudeMonitor.ca

As of April 13, two weeks after the Arkansas spill, Crude Quality Inc. listed Wabasca Heavy as dilbit under its Canadian Crude Quality Monitoring Program, which collects data on different types of Canadian crude oil and posts it online.

In an interview on April 12, Lywood said Wabasca Heavy is "primarily a heavy and extra-heavy crude oil from a bitumen-bearing formation that does contain some diluent to meet pipeline specifications." He said he included Wabasca Heavy in the dilbit category because it competes in the dilbit marketplace, and because Alberta's energy agency considers it dilbit. He said he "communicates regularly" with Cenovus and Canadian Natural Resources, who he said did not object to his decision to label Wabasca Heavy as dilbit on the website.

However, the day after that interview with InsideClimate News, Lywood moved Wabasca Heavy out of the dilbit category. It is now listed in the "Heavy Sour – Conventional" crude oil category on the CrudeMonitor.ca website.

April 18 Screenshot of Wabasca Heavy listed as conventional/Source: CrudeMonitor.ca

Lywood said in an email that this reporter's questions about the Wabasca Heavy stream were "the principal driving force behind the change" and that he "originally made an error" in listing Wabasca Heavy as dilbit. He also said he obtained new information from Canadian energy officials over the weekend that differentiates the production of Wabasca Heavy from unconventional dilbit production.

That new information confirms that Wabasca Heavy is "unquestionably conventional production," Lywood said. He declined to provide the documents, citing a confidentiality agreement.

"I'm not saying that [Wabasca Heavy] does not contain bitumen," he said via telephone Tuesday. But he said the definition of what is and isn't bitumen is "arbitrary" and subjective to the person or group defining it. There are hundreds of varieties of what could be called dilbit, with varying gravities and degrees of bitumen, he said.

Cenovus Energy, one of the main producers of Wabasca Heavy, told InsideClimate News that Wabasca Heavy is not bitumen. "It is not produced from oil sands operations, although it does come from northern Alberta," Rhona DelFrari, a Cenovus spokeswoman, said in an email.

DelFrari said the Cenovus Material Data Safety Sheet that Exxon provided the EPA—which places Wabasca Heavy in a group of dilbit blends—is "a generic document that refers to all heavy oil that needs diluent mixed with it. It lumps several types of oil together."

A spokesperson for Canadian Natural Resources, the other big Wabasca Heavy producer, declined requests for comment.

The EPA said the April 10 letter from Exxon won't change how the agency approaches the Arkansas pipeline spill cleanup.

"We have not put any new procedures in place for the cleanup," Nicolas Brescia, the federal on-scene coordinator in Mayflower, said in an interview. "The cleanup techniques and procedures are consistent with typical heavy crude oil spills."

When asked if the EPA was concerned that the bitumen might sink and accumulate under water as it did in Michigan's Kalamazoo River in 2010, Brescia said that samples of sediment will soon be taken "to see if there's oil that dropped out."

He said the EPA is coordinating that effort through the Arkansas Department of Environmental Quality, which is also taking water samples in a marshy cove of Lake Conway where some of the oil spilled.

Swift, the NRDC attorney, said that how the Wabasca Heavy responds in water over time will help put an end to the debate over whether it's diluted bitumen.

"When the crude that has hit the water, is it really floating, or is it sinking? That'd be a final check on all this noise."