"Discovered" is a pretty flexible term. Geologists will only go out to look for the stuff if there's an expected return on extracting it. Similarly, "reserves"

"Discovered" is a pretty flexible term. Geologists will only go out to look for the stuff if there's an expected return on extracting it. Similarly, "reserves" means stuff that (a) we know about and (b) that we think can be extracted economically. In effect, it's tautological to say that we'll "use the stuff until it runs out", because at whatever point we stop using it there'll still be some left that's just slightly too difficult to extract (given, hopefully, alternatives and taxes) for it to be worth it.