If renewables can produce relatively cheaper energy, it won't make sense to use oil or gas for anything other than chemical synths. And as other commenters have

If renewables can produce relatively cheaper energy, it won't make sense to use oil or gas for anything other than chemical synths. And as other commenters have said, once hydrocarbon sources run out we can just build the required molecules from a hydrogen base. Reminds me of clandestine chemists who can use clove oil to synthesise MDMA because other precursors are closely watched. As long as you have the right atoms, the only issue to building a new molecule is energy and time.