The amount of oil (or for that matter, any other resource) isn’t a fixed amount. If the price can be made to rise to a very high level, the quantity that can be extracted will also tend to rise–in fact, by a rather large amount. The “catch” is that wages for the vast majority of workers don’t rise at the same time. As a result, goods made with high-priced oil soon become too expensive for workers to afford, and the economy falls into recession. The result is prices that fall below the cost of production. Thus, the limit on oil supply is not the amount of oil in the ground; instead, it is how high oil prices can rise, without causing serious recession.