Obama. Just joking, kind of.
The amount of oil in a reserve is guess-i-mated from a series of exploratory wells drilled strategically but spaced apart. By luck (or lack thereof) you can drill next to a deposit and report dry, where 1,000 feet away you might hit a gas pocket and have a gusher. These wells are expensive to drill, and no one is going to drill hundreds of them just to tell you there is a lot of oil. They will only drill when they can reasonably expect to extract the oil they find and make some money (actually a lot of money).
Obama's drilling policy has made it uneconomical for developers to explore in the USA, because there are no guarantees they can get a permit for the production wells. We are now paying for that policy at the gas pump.
In addition to the oil in the Bakken Formation, there is also a huge reserve of natural gas in the Appalachian Mountains recently discovered and being exploited in Pennsylvania. Both ‘discoveries’ have huge financial impacts, especially finding a gigantic gas reserve so close to the east coast population zones.
Maybe Americans can let their politicians know that until ‘green’ technologies become economically feasible, we should be exploring and drilling.