Bunker fuel is what's left after crude oil has had all the lighter fractions cracked off: Small molecules like those in propane, naphtha, gasoline for cars, an

Bunker fuel is what's left after crude oil has had all the lighter fractions cracked off: Small molecules like those in propane, naphtha, gasoline for cars, and jet fuel have relatively low boiling points, and they are removed at the start of the fractional distillation process. Heavier petroleum products like diesel fuel and lubricating oil are much less volatile and distill out more slowly, while bunker oil is literally the bottom of the barrel; in oil distilling, the only things denser than bunker fuel are carbon black feedstock and bituminous residue (asphalt), which is used for paving roads and sealing roofs. https://en.wikipedia.org/wiki/Fuel_oil https://en.wikipedia.org/wiki/Fuel_oil