Some geologists believe that a geological formation under the Dakotas, Montana and Canada known as the "Bakken" could contain between 200 and 400 Billion barrels of oil.
http://www.post-gazette.com/pg/06095/679602-28.stm
The USGS has more reserved estimates but notes the likely presence of many Trillions of cubic feet of natural gas..
http://energy.usgs.gov/
Synthetic fuels can be manufactured from coal...including gasoline. The U.S. has the largest proven fields of coal in the world as well.
Then there is the possibility (though still debated) of abiogenic production of oil. Some scientist are investigating the possibility that certain fields of hydrocarbons are actually refilling. Several of these fields show that the hydrocarbons are developing much too deep in the earth's crust to come from biological sources.
The Point being....oil will not be the downfall of the U.S.
Politics and policy will...the ending of our rights as we know them, the suspension of liberty.
I have to hope our Unique set of checks and balances will find a way to hold it all together because number 8 aint looking so good.