Must of Canada's oil comes from the tar sands. Although this sources is plentiful, it is also very dirty and expensive to extract. According to the Economist magazine,

"Canadian crude is dirty. Just over half the country's oil comes from tar sands, a mixture of water, sand, clay and bitumen - an extremely dense and thick form of petroleum, which usually must be melted before it can be extracted and refined. It takes up to four barrels of water to generate one barrel of tarsands crude, and 20% of Canada's natural gas (a clean fuel) is used to produce oil (a dirty one). Mining the sands also strips forest and creates vast ponds of toxic byproducts. According to the America's Environmental Protection Agency (EPA), producing Canadian tar-sands oil generates 82% more greenhouse-gas emissions than does the average barrel refined in the United States."

Source: Economist Magazine, August 7th 2010, Page 35, Tarred with the same brush"Tar Sands Oil Reserves" has a significant impact, so an analyst should put more weight into it. "Tar Sands Oil Reserves" will have a long-term negative impact on this entity, which subtracts from the entity's value.