The important magic is that carbon and hydrogen given enough pressure will form hydrocarbons (basically as dead plankton deep below the tectonic plate did), so

The important magic is that carbon and hydrogen given enough pressure will form hydrocarbons (basically as dead plankton deep below the tectonic plate did), so doing it the brute force way is simple, but very energy intensive. Split CO2 to CO and O, split H2O to H2 and O. CO + H is syngas, used as feedstock for synthetic fuels. Then using a metal catalyst (iron or cobalt for example) and heat and pressure good old "organic chemistry" does the rest, various hydrocarbons form (and water as residual by product). This reaction itself gives off heat, so at least this part doesn't need that much energy. https://en.wikipedia.org/wiki/Synthetic_fuel https://en.wikipedia.org/wiki/Synthetic_fuel https://en.wikipedia.org/wiki/Fischer%E2%80%93Tropsch_process#Reaction_mechanism https://en.wikipedia.org/wiki/Fischer%E2%80%93Tropsch_proces...