Canada is the largest source of U.S. oil imports, supplying 21 percent of U.S. petroleum imports in 2009.[0] But Canadian oil reserves, though abundant (the second largest in the world), are mainly composed of oil sands. Oil sands are a heavy, unconventional oil that exist in a semi-solid or solid phase. They need to be heated or diluted with hydrocarbons to flow like conventional crude oil. Because oil sands require additional processing, compared to conventional crude, they require higher oil prices to be economic. Their production also results in slightly higher levels of greenhouse gases than conventional crude oil, also owing to the additional processing required.[ii]
