Since the early days of the petroleum industry, some physical properties of crude oil were used to define characterization factors for classification of crude oil with respect to hydrocarbon types [4] as shown in Equation 8.
where: Tb = volume, or mean average normal boiling point in R (degree Rankine) and SG = specific gravity at 15.6°C (60°F). To calculate KUOP or KW,volume average boiling point (VABP) or mean average boiling point is used, respectively. Depending on the value of the Watson characterization factor, crude oils are classified as paraffinic (Kw = 11-12.9), naphthenic (Kw =10-11), or aromatic (Kw <10).
Another parameter defined in the early years of petroleum characterization is the viscosity gravity constant (VGC). This parameter depends on viscosity expressed in Saybolt Universal Seconds (SUS) and specific gravity. According to a standard method (ASTM D2501), VGC can be calculated at a reference temperature of 100°F as follows in Equation 9:
According to this CI scale, all n-paraffins have a CI value of 0, while cyclohexane (the simplest naphthene), has a CI value of 50, and benzene has a CI value of 100. Using the CI values, crude oils can be classified as follows:
| Table 2.1 | |
| paraffinic | CI<29.8 |
| naphthenic | CI<57.0 |
| aromatic | CI>75.0 |
