The Germans have done a lot of work on straight vegetable oil running current model COTS diesel engines.

Imagine hydroponic greenhouses in some wasteland growing canola, for example.

This is a little different than waste vegetable oil aka biodiesel mostly made from old fryer oil and needing lots of processing and filtration, but roughly the same concept.

Obviously all the oil underground came from plants initially, you can skip a lot of trouble and harvest your sunflower seeds and rapeseed seeds right on the surface and skip all that "bury it for a few million years then tunnel down and dig it up"