For example, your posted article: The Energy Information Administration (EIA) estimates it at 503 billion barrels....
The USGS article: Reston, VA - North Dakota and Montana have an estimated 3.0 to 4.3 billion barrels of undiscovered, technically recoverable oil in an area known as the Bakken Formation. (for those of you scoring at home that's a difference of about 500 BILLION barels.
It would appear that the article you posted bears no relationship to the article linked.
I don’t know....I got the text I posted in an email, went to the link, saw it had most of the same text and pasted it.
http://www.eia.doe.gov/pub/oil_gas/natural_gas/feature_articles/2006/ngshock/ngshock.pdf
Maybe the difference is between what is there and what is technically recoverable....