So, for the sake of argument, let’s assume at some future date and time, light crude oils have become rare enough that the market price is $1000 a barrel (in 2014 dollars).
Gasoline and diesel and kerosene are now so expensive to be impractical for most uses.
How could we keep a network of ships, planes, trains, and automobiles running?
Well, all of those things could use CH4 instead of liquid fuels. Interestingly, liquid methane has a better energy : mass ratio than kerosene. I know you could run ships, trains, and trucks using the liquid methane, and I suspect that airliners could be designed to use liquid methane as well. (this would cause some significant engineering problems but it’s viable on paper)
For right now, natural gas is dirt cheap, but what about after all the easily recovered reserves are gone in another century?
In principle, could you build giant nuclear plants on a large and efficient scale that would produce steam for efficient electrolysis of water and to heat the reaction vessels performing the Sabatier reaction?
You would obtain CO2 by either burning (now somewhat rarer) coal or natural gas in peak load electric plants. (the nuclear plants are for base load). You trap all the exit gas, purify the CO2, and liquify it.
You then crack water using energy from the nuclear plant (either via electrolysis of steam or possibly more…directly), and react the C02 and hydrogen to get methane, which you then liquify for use.
Individual passenger cars would use compressed natural gas instead.
You could even separate C02 from the atmosphere directly via refrigeration for a “net zero” source of fuel.