Diesel fuel is made from crude petroleum. Small amounts of sulfur, nitrogen and oxygen (so-called heteroatoms) are present in this fuel. When heteroatoms are bound into molecular structures with carbon and hydrogen, the resulting compounds are not characterized as hydrocarbons. Typical examples of nonhydrocarbon compounds found in diesel include dibenzothiophene (a sulfur compound) and carbazole (nitrogen compound).
The distillates heavier than kerosene derived by atmospheric distillation of crude oil are the gasoil streams. Gasoils are the main blending components for diesel fuel production. Additional types of gasoil can be derived from cracking processes. Gasoils from hydrocrackers have very good ignition quality properties, while gasoils from thermal and catalytic cracking have poor ignition quality properties. Depending on the type of crude oil used, the properties of the produced gasoil are different.