For several decades now the crude oil or petroleum industry has classified the raw crude by the location from which it was extracted. In other words, oil is classified by geographic region. However, all oils from a particular region are not created equal. Further classification of petroleum, derived from the density of the raw petroleum (API gravity) and its various non-hydrocarbon components (especially sulfur), is then added to the geographic designation. The end result of all this classification helps determine the price of a specific barrel of crude as well as how much demand there is for that particular oil.
In general, if the crude oil contains high levels of sulphur the petroleum classification is termed ‘sour, if it has relatively low levels of sulphur the petroleum classification is termed ‘sweet'. If the raw petroleum is of a high density then the petroleum classification is termed ‘heavy' and if it is of a low density the petroleum classification is termed 'light'. Density of oil is determined by the length of the hydrocarbons it contains. If it contains a great deal of long-chain hydrocarbons, the petroleum will be denser. If it contains a greater proportion of short-chain hydrocarbons it will be less dense. Besides chain length, the ratio of carbon to hydrogen also helps to determine the density of a particular hydrocarbon. The greater the amount of hydrogen in relation to carbon, the lighter the hydrocarbon will be.Less dense oil will float on top of denser oil and is generally easier to pump.
The hydrocarbons in crude oil can generally be divided into four categories:
- Paraffins: These can make up 15 to 60% of crude and have a carbon to hydrogen ratio of 1:2, which means they contain twice the amount of hydrogen as they do carbon. These are generally straight or branched chains, but never cyclic (circular) compounds. Paraffins are the desired content in crude and what are used to make fuels. The shorter the paraffins are, the lighter the crude is.
- Napthenes: These can make up 30 to 60% of crude and have a carbon to hydrogen ratio of 1:2. These are cyclic compounds and can be thought of as cycloparaffins. They are higher in density than equivalent paraffins and are more viscous.
- Aromatics: These can constitute anywhere from 3 to 30% of crude. They are undesirable because burning them results in soot. They have a much less hydrogen in comparison to carbon than is found in paraffins. They are also more viscous. They are often solid or semi-solid when an equivalent paraffin would be a viscous liquid under the same conditions.
- Asphaltics: These average about 6% in most crude. They have a carbon to hydrogen ratio of approximately 1:1, making them very dense. They are generally undesirable in crude, but their 'stickiness' makes them excellent for use in road construction.
When considering the petroleum classification it is important to consider the fact that the overall classification will have an effect on the value, not just the physical properties. For example, petroleum with a geographical classification from one region of the world may be expensive to transport to another region of the world regardless of the suitability of the raw petroleum as an overall substance. In general, lighter crude commands a higher price because it contains more hydrocarbon chains that can be easily refined to make gasoline and diesel, which are in high demand. The lower the sulphur content, the higher the price as well because low-sufur, sweet crude requires less refining.
Classification of petroleum also indicates the best use for a particular field of petroleum. One oil type is not necessarily “better” than another, but rather the different types are useful in different applications. Light crude oil is preferable for refining into gasoline as it produces a far higher yield than heavy. In a similar fashion, sweet petroleum is often more desirable than sour petroleum as its use will cause far less impact on the environment in the form of harmful emissions as it is burned. These basic classifications of petroleum are further enhanced by a full molecular description gained through a crude oil assay analysis.