The world economy runs on cheap transportation. Within a continent, we can replace gas with electricity if we have a good power source. Between continents, we need to fly at jet speeds, or commerce becomes much slower. And that means a power source that is not only portable, but produces a huge amount of thrust to keep a very heavy object in the air. what will replace the portability and explosive power of petroleum-based jet fuel?
Synthetic fuels could always be employed – but don't expect cheap air-fares. Hydrogen has been proposed but it has so little energy for the stuff you have to drag along to use it.
The "market" always adjusts to conditions – but it is discriminatory because sooner or later the more valuable something becomes the smaller the population who can afford it becomes.
Deep in the Earth are coal caves.I'm seeing that granite turns to coal at such a depth.Water flows in at a high pressure.The temperature might be high and I suspect radioactivity.The water in the cave is now oil etc.(my findings)
Well, most of our electricity is currently generated from oil.
Anyway, I'd think hydrogen. Ref above, the equipment to use liquid or gas hydrogen for combustion is not heavy & it is far more explosive than jet fuel, which is actually the problem. You can check out the NASA work with solar, but that's unreasonable for high speed air transport with heavy loads. There were early experiments with nuclear powered bombers, but those were abandoned because the large heavy reactors & shielding were problematic, and you really don't want one of those things crashing, but technology has & will continue to move forward to a point where that may be an option. I guess in the shorter term you could operate on highly refined alcohol from sugar/soy/corn/etc. Piston aircraft could start that now if you can get the economics in line alongside ethanol for cars. Jets are more of a problem, but you could generate a usable fuel fairly easily I think, it's just kerosene anyway.
Perhaps we'll come up with a kind of vegetable oil that a jet could burn, but we would need lots of canola for that, let me tell ya.
None of us will be alive when "the oil runs out", so why do you even care? Airplanes as we know them know won't even exist..hell, for that matter, PEOPLE might not even exist.