What would humanity do when all the oil reserves run out?

Answer 1

The same thing we've done when other scarce resources are depleted - find something else to substitute for it, or find a different way to do what we were doing before hand. Offhand, I'd bet that we'll increase our dependence on nuclear power. There are several new designs for reactors that look promising.

There is also a lot of research into alternative energy, like solar, wind, tidal, and biomass. Hydrogen may become a popular fuel if we can figure out a way to produce it more cheaply, and to store it safely. Alcohol from corn or other crops is possible, although I'm not convinced that it will become viable economically.