In the old days, the oil came gushing out of the ground, often under its own pressure. Recoverable reserves using the technology of the past left 2/3 of the oil unrecoverable. What we are now recovering, due to newer "fracking" techniques, is the oil that was originally left behind by the old technology.
But there is only so much oil down there. Once it is gone, its gone. Or what little is left behind is so expensive to recover, its not worth it.
The oil presently recoverable by fracking costs about $40 a barrel to recover. So, when the price of oil falls below that level, you make no money extracting it. I think part of the reason why we have so many embargoes on oil producers such as Venezuela is, if they were allowed to freely sell their oil, our Permian Basin producers of fracked oil would be driven out of business.