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.
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.
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.
The "gray area" between the two types above. Both paraffins and naphthenes are present, as well as aromatic hydrocarbons. Most crudes fit this category.