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Heavier crudes are, by definition, more viscous. The also contain greater amounts of sulfur, which must be removed because of the adverse impact of sulfur on various catalysts used in the refining process. The sulfur content of fuel is also controlled because of its impact on particulate generation. The sulfur content of oils entering U.S. refineries has been increasing since 1985, according to DoE. The additional refining activity, not to mention the greater energy cost of extracting such heavier crudes, all add increased GHG emissions from extraction and refining activities.
A recent assessment of this impact can be found at http://pubs.acs.org/doi/abs/10.1021/es1019965. To date, I have been unable to find any criticisms of the analytical methodology employed, but I would anticipate that there will be some in the near future.
The National Wildlife Federation has also done an assessment of the impact from tar sands. Their report can be found at http://www.nwf.org/News-and-Magazines/Media-Center/News-by-Topic/Global-Warming/2010/10-12-10-Tar-Sands-Tour.aspx.