Wednesday, March 27, 2013

ATHABASCA TAR SANDS


If you’ve followed the news, you’ve probably heard a fair amount about the processing of “tar sands” in Canada, as they have become both a major supplier of oil to the U.S. and controversial for many environmental reasons.

Without taking a specific side of the political debates in this post, I thought it would be interesting to cover the geology of this deposit and what exact...ly it is, since that doesn’t get discussed very often. If nothing else, it probably will lead to some lively comments!

The key ingredient in the Canadian tar sands is a material called “bitumen”. Another word for it is asphalt, which is probably more familiar to people. Bitumen is basically a version of petroleum that is solid at room temperatures. It’s fairly sticky, it does flow if you heat it, but it’s like a very thick sludge. In road construction, asphalt is often used as the glue that holds the road together. If you’ve stepped on a recently paved road on a hot day and felt like your feet might stick to the road…that’s asphalt. That’s what’s being mined here.

The bitumen in Canada is a product of several geologic events. First, rocks were laid down and buried that could act as source rocks; carbon-rich rocks that give off oil when heated called kerogen. There is some debate about what exactly the source rocks were, but they must be buried somewhere. Later, on top of the source rocks, an estuarine system developed. An estuary is a place where a river meets an ocean. The river delivers sand and mud, building up thick piles of sedimentary rock, including lenses of sand. These rocks are, in places, known as the McMurray formation, and the sandy layers are the rocks that wind up hosting the “oil”. Next, carbonate rocks were formed on top of the estuarine sediments. The carbonates are important because they’re impenetrable; oil can’t flow through them easily.

Finally, to form oil, something major happened; the Rocky Mountains formed, impacting this area in the late Cretaceous (~65 million years ago). The building of those mountains heated the source rocks to create oil, and created pressure that forced the oil to migrate. The oil moved from the source rocks into the estuarine sands, which are good places to trap oil. The oil was trapped there, finally, by the carbonate rocks above, which it couldn’t migrate through.

All of these are normal processes in oil reservoirs, but something different happened here. The oil in these sediments was shallow, close to the Earth’s surface, and held at low temperature, mostly below 80°C. At these temperatures, bacteria can survive, and bacteria have evolved to use many of the chemicals in oil as energy sources. The bacteria must have gone wild; they ate enormous amounts of oil as it migrated into these rocks.

The stuff left over is, well, leftover waste. It was the stuff too complicated or too difficult for the bacteria to consume. Oil is made up of many chemicals, some of them easy to use, some not. The not-so-easy-to-consume chemicals were left in the rocks, which built up into thick piles of sticky sludge. Furthermore, that sludge is mixed in with the sandy estuary rocks; the sludge and sand stick together like road-building materials. When these rocks are exposed at the surface, they can be sticky, sometimes seeping thick viscous carbon-rich oozes, but they’re really solid rock.

That’s the stuff being mined as tar sands and converted to gasoline in Canada. The process is like taking a freshly-paved, blacktop road, grinding up the material, and extracting oil from it. Many people in the Midwestern U.S. right now are running their automobiles using gasoline created from the leftover products of whatever bacteria didn’t eat in these oil reservoirs, but mankind has figured out a way to process industrially.


References: Zhou et al., (2008) Biodegradation and origin of oil sands in the Western Canada sedimentary basin
http://www.springerlink.com/index/P4277621627L47K7.pdf

Musial et al., (2011), Subsurface and outcrop characterization of large tidally influenced point bars of the Cretaceous McMurray Formation (Alberta, Canada)
http://www.sciencedirect.com/science/article/pii/S0037073811001722

Geologic features of the Athabasca oil sands
http://www.ramp-alberta.org/river/geography/geological+prehistory/mesozoic.aspx

Image credit: UT Austin Bureau of Economic Geology
http://www.beg.utexas.edu/indassoc/dm2/qcl_news_2010.htm

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