"Human Ward glared at their fellow Deathworlders, and briefly thought about geologically-rendered dinosaur fat"

Actually, it wasn't just the fat, and while some of it was from the dinos, the majority of oil came from plants. Places where ancient peat bogs once were, is where the largest amount of crude, especially fine crude, is located. In places where there is oil shale, that is to say, shale stone that had not yet gone through enough pressure and time to squeeze out the organic molecules, you will find tons of plant fossils, fossilized trees and leaves, and some smaller prehistoric animals. But the majority of oil came from plants, since plants capture the most carbon and return the most carbon to the planet.