The giant oil sands of Alberta finally have a date. And it’s a lot older than anyone expected.

David Selby and Robert Creaser, from the University of Alberta, recently put an age of 112 ± 5.3 million years ago for the migration and accumulation of oil in the Alberta oil sands – a date over 60 million years earlier than previously thought.

Although, the date isn’t the first to be done on the sands, it’s the first in the world to be done with such accuracy.

“This has only ever been done on a relative scale before with something like fossil dating,” says Selby, a postdoctoral fellow and lead author of the study. “We are quite excited by it,” he says.

Dating the formation of oil is no easy task. This is because oil and gas forms from a variety of organic material and also tends to migrate through rock. In fact, as the oil migrates it either winds up escaping to the surface or is trapped and forms petroleum fields like in the western Canadian sedimentary basin in Alberta. This process tends to remove any memory of where and when these fields first formed.

As a result, “there was an immense debate on when the oil sands formed,” says Selby. This has also left explorers looking for oil with little information on what type of rocks might contribute to petroleum fields.

To tackle this problem, Selby and Creaser decided to use a naturally occurring chronometer in the form of isotopes from two elements found in the Earth’s crust – rhenium and osmium. This chronometer is based on an unstable rhenium isotope, rhenium-187, which decays into osmium-187. This rate of decay is constant allowing researchers to use it as a geological clock.

The first question for Selby and Creaser therefore became whether they could detect rhenium and osmium in oil. For an answer, they turned to a type of rock termed black shale, which is the source of oil as it migrates and matures into a petroleum fields, says Selby.

Upon examination of the shale, Selby and Creaser found 3 to 50 parts per billion rhenium and 25 to 290 parts per thousand osmium. Since oil picks up rhenium and osmium as it migrates through the shale, the researchers were then able to examine rhenium/osmium ratios in oil to accurately date it.

What Selby and Creaser found places the creation of the Alberta oil sands at a much more ancient date then previously suggested (researchers had thought the oil sands formed during the Late Cretaceous Laramide Orogeny, ~ 60 million years ago.) The findings also suggest the oil formed from a single source.

“This helps answers some huge questions,” says Selby, “about when the oil sand formed and how they were trapped.” The results of the study are published in the May 27, 2005, issue of Science.

In an accompanying comment piece in the same issue, Bruce Schaefer, from Monash University, Australia, says these findings are important for petroleum exploration.

“For petroleum explorers, knowing the origin of hydrocarbons in a sedimentary basin places constraints on where they might be able to accumulate, or whether they are able to accumulate at all. With oil exploration drillholes costing multiple millions of dollars, every piece of data informing site location is of immense worth,” writes Schaefer.

Selby isn’t quite so sure of the immediate impact to petroleum exploration, but says “we are beginning to look at other oils from all over the world.”

So, the technique may help some day. But right now, “it’s very very new,” says Selby.

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