The oil reserves you refer to were expensive to get at with horizontal drilling when the world oil price was $10 US per barrel but now at 70+ a barrel it is easily profitable. The cost for extraction from the Bakken reserve is around 35-40 dollars a barrel, there is no excuse for not using this resource other than politics...
Mick
Say you are hungry, starving, and while walking you find an apple tree, full or ripe apples. There are apples you can reach, some of them you can reach by jumping, and for the rest you would need a ladder, conveniently lying besides the tree.
Even when you can easily get apples by jumping, chances are you wont jump for apples unless there are no more apples at a hand's reach. You maybe will jump for them when the remaining apples at hand's reach are either rotting or the quality of the ones at a higher level are worth a jump, compared with the ones at hand reach.
Obviously, the ladder will only be used when there are no more apples either at hands's or jump reach.
That is, even when an apple is worth a jump, you won't jump for apples when there are apples easier to get. The same goes with oil, even when there are reserves that can be extracted at a profit, they won't be until the reserves that have less extraction costs are exhausted.