Despite a wide variety of crude oil found in different parts of the earth, the elemental composition of most crude oils changes in narrow ranges, as shown in Table 2.2.

With such narrow ranges of change in elemental contents, elemental composition does not have much utility for classification of crude oil. Instead, variations in hydrocarbon composition (paraffins, naphthenes, and aromatics) are used to classify crude oils, using a ternary diagram, shown in Figure 2.2. Each apex of the triangle represents 100 percent weight of the corresponding compounds, and 0% of this particular type of hydrocarbons on the side of the triangle across from the apex. For example, the side at the bottom of the triangle (across from the apex of 100% aromatics) represents binary mixtures of paraffins and naphthenes.

Figure 2.2. A ternary diagram for classification of crude oils

If you need to refresh your memory on reading ternary diagrams, you may check “Reading a Ternary Diagram”(link is external), or consult other sources. The list below shows the six classes of crude oil that are defined using a ternary diagram. These classes are shown as areas on the ternary diagram for paraffins, below. It is generally accepted that Class 1 (rich in paraffins) represents the most desirable type of crude oil because refining these crudes would readily lead to high yields of light and middle distillates that constitute the fuels such as gasoline, diesel fuel, and jet fuel which are in high demand. Extensive refining would be required to produce high yields of distillate fuels from aromatic crudes (e.g., Class 4-6). Class 1 crudes tend to have high °API and low sulfur contents and tend to be more expensive than the other types of crude oils.

Ternary Classification of Crude Oils

1.      Paraffinic Crudes
paraffins + naphthenes > 50%
paraffins > naphthenes
paraffins > 40%

2.      Naphthenic Crudes
paraffins +naphthenes > 50%
naphthenes > paraffins
naphthenes > 40%

3.      Paraffinic-Naphthenic Crudes
Aromatics < 50%
paraffins < 40%
naphthenes < 40%

4.      Aromatic-Naphthenic Crudes
Aromatics > 50%
naphthenes > 25%
paraffins < 10%

5.      Aromatic-Intermediate Crudes
Aromatic > 50%
paraffins > 10%

6.      Aromatic-Asphaltic Crudes
Naphthenes < 25%
paraffins < 10%