It's full of sulfur and very heavy.
"Buried just beneath a layer of muskeg and forest in northern Alberta, Canada, lies a 50,000 square mile reservoir of heavy crude oil, possibly holding 2 trillion barrels of recoverable oil. These bitumen deposits require a lot of effort to extract, recover, and pre-process before the oil can be sent to conventional refineries. Most of the current extraction process takes place in open-pit mines, with massive machinery scraping up the tarry sandstone and moving it to facilities for processing. As the name "tar sands", or oil sands, implies, the heavy crude is found mixed in with sand, clay, and water, which must be removed, then the heavy crude must be "upgraded" to reduce viscosity and improve quality."
www.theatlantic.com