Oil will never be all gone, it will just be found in smaller pockets, greater depths, grosser forms (thicker,  more sulphur,"oil sands", née tar sands...) and -- especially -- in fields that have given up a lot of oil already. Oil occupies pores in the rock (never a pool!), and it's hard to get all the grease out of something like that. First they let gas pressure push it out, then they pump stuff in to raise the pressure, or pump oil out as it drains into a hole, then they start heating the ground to make the stuff less viscous, or pump in detergent, or...

So the price rises, and people are forced to use something else, or just use less energy -- even though there's still a zillion tons of crud left in the ground.

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