It has become a commonplace to term Canada not just the holder of the world's third-largest oil reserves, but the U.S.'s biggest source of oil imports, supplying a quarter of these needs, well over 2 million barrels a day.
The reality is a bit more complex. The U.S. consumes over 19 million barrels a day. The country's net imports come to about 8.3 million barrels a day. Meanwhile, Canada currently produces about 3.5 million barrels a day. However, Canada consumes about 2.3 million barrels a day itself. This leaves its net exports a mere 1.2 million barrels – with the million-barrel difference covered by the country's own imports from Saudi Arabia, Africa and Venezuela. In short, Canada meets a quarter of the U.S.'s energy needs only by meeting its own needs with oil coming in from more traditional producers, and indeed the exact same countries Canadian oil is supposed to delink American energy consumption from. Putting it another way, Canada's production above its own needs supplies 14 percent of U.S. imports, only a bit over half of the more commonly cited figure, and that the situation appears otherwise is due to the Canadian energy market being "subsidized" by cheaper oil imports from elsewhere.
Certainly Canadian production is widely expected to grow, with one EIA forecast positing its expanding from about 3.5 million barrels a day in 2010 to the area of 5 million barrels in 2020-2025, and 6-7 million barrels a day in 2030-2035. Additionally, Canada's consumption is likely to grow at a much slower rate than that during this time frame. Of course, it remains to be seen that production will actually reach these levels, and even were they to do so, Canada might well seek to meet more of its own needs from domestic energy production. Still, this would likely leave a growing surplus available for export.
At the same time, there is considerable optimism about the U.S.'s need for oil imports actually falling between now and then, quite a bit of it having to do with a report from British Petroleum this year (highlights from which were reported in the Guardian earlier this year). The report envisages the U.S.'s consumption falling (as oil use becomes more efficient), and its production of its own liquid fuels increasing (as the long-promised production of shale oil takes off, and biofuels substitute for crude oil). Yet, there is at the very least room for skepticism about the prospect of an imminent shale boom. It is also worth remembering that even if Canadian production was voluminous enough to meet all of the U.S.'s needs, the reality is that Canadian oil production will still be just part of the global pool – so that prices and supplies will still be subject to fluctuations caused by events elsewhere in the world. It is worth noting, too, that the production of oil from tar sands depends on the use of natural gas – another fossil fuel which has grown more expensive in recent years, and which remains concentrated overwhelmingly in Russia and the Persian Gulf region. (And of course, where greenhouse gas emissions and other environmental effects are concerned, non-conventional oil of this type is significantly worse than regular, liquid oil.)
Just as before, the only way to really end U.S. dependence on problematic fossil fuel exporters is to end its dependence on fossil fuels, in favor of energy production from other sources. For the time being, this would mean trading natural resources of one kind for another – given the role of rare earth elements in renewable energy technology, for instance – but, politically and ecologically, that would still be an improvement over burning a constant, massive flow of hydrocarbons.