There are two basic types of marine fuels â distillate and residual. A third type is a mixture of these two, commonly called âintermediateâ. Distillate fuel is composed of petroleum fractions of crude oil that are separated in a refinery by a boiling or âdistillationâ process. Residual fuel or âresiduumâ is the fraction that did not boil, sometimes referred to as âtarâ or âpetroleum pitchâ.