The credit rating agency says oilsands players need prices of between US$80 and US$100 a barrel to be economically viable.
On the other hand, producers in the Bakken — a shale formation that underlies parts of North Dakota, Montana, Saskatchewan and Manitoba — need prices of between US$60 and US$80.
Bakken oil is of higher quality than bitumen from the oilsands, which requires upgrading before it can be refined into fuel.
Bakken oil can also be more easily transported to market by rail, as opposed to the oilsands, which are suffering from pipeline constraints.
And the upfront capital costs in the oilsands are higher than they are in the Bakken.