Canada has deliberately made itself into an oil supplier primarily for the U.S. The NAFTA agreement cements this relationship. While Harper talks about finding new markets for our oil, the eastern provinces of Canada import almost all their oil from abroad. It would make much more sense to have oil from Alberta flow to Ontario, Quebec and the Maritimes rather than down south as far as Texas!
Obama and Stephen Harper at APECChinese president Hu Jintao has welcomed Harper's move to market oil in China. He has invited Harper to visit China next year. However, to ship large quantities of oil to China a pipeline from Alberta to Kitimat British Columbia would need to be approved. There will be considerable opposition to building that pipe line from environmentalists and first nations groups.
Many environmentalists want to block further development of Tar Sands oil. They claim that production of Tar Sands oil involves the emission of far too many greenhouse gases. First nations in the area also have complaints about the effects of production on them. Given the dependence of the global economy on fossil fuels the environmental concerns will in the end not likely be powerful enough to stop further increases in production. For more see this article.