Although it's clearly not as large as the 200 billion figure some people were hoping for, it's still not trivial:
This is a lower figure than I had guessed. However, it is 24 times more than the 1995 estimate of 150 million barrels and 6 times more recent quotes of 600 million barrels... Proven reserves increases by almost 3 billion barrels. The 95% figure was 3063 million barrels of oil. The old proven reserves was 150 million barrels. So the US proven reserves number goes up to 24 billion barrels, a 15% increase. Adding almost 2 years to the 12 year life of US reserves based on proven reserve calculations. The Bakken is now the largest probable reserve in the United States outside of Alaska. It is one third the size of Alaska's ANWR.(Via Caiazza's.)
