Once the crude oil has been fractioned, the fractions can be changed into different types by cracking (breaking large hydrocarbons into smaller ones), unification (combining small pieces to make larger hydrocarbons) and alteration (rearranging different pieces to make different hydrocarbons). Cracking can be done using heat, where the hydrocarbons are heated to very high temperatures until they break apart, or by using a catalyst to speed up the cracking reaction.
Coking, a sub-process of cracking, is where residuals are heated until they crack into heavy oil, petrol and naphtha. A pure carbon residue (coke) is left behind, which is then cleaned and sold.
Unification uses catalytic reforming. A catalyst is used to combine naphtha into aromatics which can then be used in making chemicals and blending petrol.
Alteration uses a process called alkylation. Molecularly light compounds are mixed in the presence of a catalyst to produce high octane hydrocarbons, which are used in petrol blends.
Putting all the processes together in a diagram, we get:
