According to the link which you yourself provide, the Bakken Formation
contains an average estimate of 3.65 billion barrels of recoverable oil.
That sounds like a lot, but when you consider that the US currently
consumes almost 21 million barrels of oil PER DAY, the oil in the Bakken
Formation would only supply America's needs for about 175 days.
The estimates you speak of showing as much as 500 billion barrels are not
really indicative of anything, since what matters is how much oil is
recoverable. If the Moon contained 100 trillion barrels of oil, that would
be nice, but it would also be useless, since we can't recover it. The US
Geological Survey estimates that, because of the nature of the rock in the
Bakken, only about 1% of the oil is recoverable, using current technology
(see link, as well as your on Wiki link).
As for the other large formation you mention, under the Rocky Mountains,
that is oil shale, not liquid oil. Oil shale is rock, which cannot be
"drilled." It has to be mined. And even then the rock doesn't contain oil,
per se, but kerogen, which can be extracted from the rock and refined into
a synthetic oil. Of course the unrefined rock itself can be burned, too,
but only as a very low-grade fuel.
The problem is that extracting and refining oil shale is extremely
expensive, much more so than drilling offshore or through permafrost in
remote Alaskan areas. Currently it is not economically viable. That's the
main reason it isn't being done now. If the price of oil continues to go
up, however, this "reserve" may become economically recoverable, as oil
sands have done in Canada (which, by the way, is where the US gets most of
its foreign oil). But the rising cost of oil will drive up the price for
everything made from oil, such as plastics, and everything that relies on
oil for production, such as food. Long before the last chunk of oil shale
could be produced from the Rockies, people will have developed cheaper,
more renewable forms of energy.
Remember, the current oil business began only about 150 years ago as a
reaction to the rising cost of whale oil as whales became scarcer.
Economics drove that technological shift, and it will eventually drive the
shift away from petroleum.