If a geologist named Gold is correct, and there is evidence that he is, the earth is still making oil so what we have now isn't all there will be.
I read Gold's theory that the Earth is loaded with oil and oil did not originate due to decaying plants and animals. I always thought that idea of oil being limited, and due to dead animals sounded too far fetched.
That guy is FASCINATING...and he gets almost no play. But I DO remember a rather lengthy NYT article about 6 - 10 years ago.
Its strange isn't it, that mankind has had to rely on earth's resources throughout history?
While cadres of bedwetters have rung the disaster bells, this old earth continues to support us. It supports some of us better than others. For instance, most United Nations member nations, and not to be redundant, most African nations.
Going back to the original question, if not the earths resources, then what.
Gold isn't the only one;
Back in the 70s at the engineering college I was attending, the Petroleum Engineering / Petroleum Geology Research Center staff/faculty were mostly saying pretty much the same things.
I never studied the details particularly (as I was over on the Mining engineering / mineralogy side), but remember reading a few papers and hearing chit-chat about it, and thought it sounded pretty reasonable at the time.