CRUDE OIL CLASSIFICATION
Appreciable property differences appear between crude oils as a result of the variable ratios of the crude oil components. For a refiner dealing with crudes of different origins, a simple criterion may be established to group crudes with similar characteristics. Crude oils can be arbitrarily classified into three or four groups depending on the relative ratio of the hydrocarbon classes that predominates in the mixture. The following
describes three types of crudes:
1. Paraffinic—the ratio of paraffinic hydrocarbons is high compared to aromatics and naphthenes.
2. Naphthenic—the ratios of naphthenic and aromatic hydrocarbons are relatively higher than in paraffinic crudes.
3. Asphaltic—contain relatively a large amount of polynuclear aromatics, a high asphaltene content, and relatively less paraffins than paraffinic crudes.

A correlation index is a useful criterion for indicating the crude class or type. The following relationship between the mid-boiling point in Kelvin degrees (°K) and the specific gravity of a crude oil or a fraction yields the correlation index (Bureau of Mines Correlation index).

BMCI = 48,640 / K + (473.6d – 456.8)
K = mid-boiling point in Kelvin degrees (Mid-boiling point is the temperature at which 50 vol % of the crude is distilled.)
d = specific gravity at 60/60°F     

A zero value has been assumed for n-paraffins, 100 for aromatics. A low BMCI value indicates a higher paraffin concentration in a petroleum fraction.
Another relationship used to indicate the crude type is the Watson characterization factor. The factor also relates the mid-boiling point of the crude or a fraction to the specific gravity.

where T = mid-boiling point in °R (°R is the absolute °F, and equals °F + 460)
A value higher than 10 indicates a predominance of paraffins while a value around 10 means a predominance of aromatics.Properties of crude oils vary considerably according to their types. Table below  lists the analyses of some crudes from different origins.