Those are reserves that are economically recoverable at current prices. The U.S. has absolutely enormous deposits of shale oil that can be (and are, right now) mined with current technology, they are just more expensive than liquid oil and so generally not economically worth it at current prices. In the event of a major geopolitical conflict that sends the price of oil skyrocketing, you can bet that the U.S. is going to tap its estimated 2.175 trillion barrels of shale oil reserves.

https://en.wikipedia.org/wiki/Oil_reserves_in_the_United_Sta...