That's called a closed loop, and it's actually vaguely feasible for natural gas: https://en.wikipedia.org/wiki/Sabatier_reaction https://en.wikipedia.org/wiki/S

That's called a closed loop, and it's actually vaguely feasible for natural gas: https://en.wikipedia.org/wiki/Sabatier_reaction https://en.wikipedia.org/wiki/Sabatier_reaction The problem is that it's not cost-effective to produce methane via the Sabatier process when you can just drill it out of the ground.