Let me ask.... over 99% of Canadian oil is exported to the United States. Today the demand is greater than the Canadian supply. Why would this oil be exported outside of North America?
I can understand Canada wanting to diversify its base of end customers. Having one customer consume over 99% of your product is a economic risk - but this would only happen if Canada build a pipeline to their east coast.
There is no sound argument (or evidence) that this oil is going to be exported. It is not exported now when it is shipped by train to the U.S. Building the pipeline would simply send the same amount of Canadian shale oil to the U.S. in a more safe and economical manner - while not increasing the amount of oil (which is fully consumed today in the U.S.).
Its the sulfur content.
All the different grades go various directions because of their suitability for different end-product processes. The stuff used to make Vaseline and medical grade products is different from the stuff used to make gasoline. For sure a significant amount of high-sulfur crude will leave the hemisphere (be exported) and a lot of the light-sweet crude will be used to refine gasoline domestically.