Crude oils vary widely in appearance and viscosity from field to field. They range in
colour, odour, and in the properties they contain. While all crude oils are essentially
hydrocarbons, the differences in properties, especially the variations in molecular
structure, mean that a crude is more or less easy to produce, pipeline, and refine. The
variations may even influence its suitability for certain products and the quality of those
products.
Crudes are roughly classified into three groups, according to the nature of the
hydrocarbons they contain.
Paraffin-based Crude Oils
These contain higher molecular weight paraffins which are solid at room temperature, but
little or no asphaltic (bituminous) matter. They can produce high-grade lubricating oils.
Asphaltic Based Crude Oils
Contain large proportions of asphaltic matter, and little or no paraffin. Some are
predominantly naphthenes so yield a lubricating oil that is more sensitive to temperature
changes than the paraffin-base crudes.
Mixed Base Crude Oils
The "gray area" between the two types above. Both paraffins and naphthenes are present,
as well as aromatic hydrocarbons. Most crudes fit this category.
Crude oils usually contain small amounts of combined oxygen, nitrogen and sulphur.
Crude oils obtained from various localities have widely different characteristics
indicating that the hydrocarbons have different properties. Nearly all crude oils will give
ultimate analyses within the limits shown below:
Classification of crude oils based on Gas Oil Ratio:
Black Oil ; solution GOR, (Rs) less than 2,000 scf/bbl
Volatile oil : solution GOR, (Rs) greater than 2,000 scf/bbl
