Crude oil consists largely of hydrocarbons, which are chemicals composed solely of hydrogen and carbon in various molecular arrangements. Crude oils contain hundreds of different hydrocarbons and other organic and inorganic substances including atoms of sulphur, nitrogen and oxygen, as well as metals such as iron, vanadium, nickel and chromium. Collectively these other atoms are called heteroatoms. Certain heavy crude oils contain less than 50% hydrocarbons and a higher proportion of heteroatoms (USEPA, 2004).
Although crude oils vary in composition, they can be divided into three main categories based on compounds. These groups are:
Light-weight components
Medium-weight components
Heavy-weight components
The following sections discuss each of these individually.
