It's not just oil, it's also coal and other integral resources that are now difficult to extract because we are at the upper level of the S-curve and efficiency has dropped.

Well, coal is the resource that's gone, but all the metals - iron, silver, aluminum - aren't depleted (they don't disappear when we use them) and are much more accessible than they were before, because we dug them out from deep underground, refined them (especially for aluminum) and concentrated them in various places. If civilization would disappear, our scrapyards are better mining locations than anything the Romans had.