The classification of crude oil can be done using many factors. It can be classified as hydrocarbon source based on physical state together with other fossil fuels.
It can also be classified based on their chemical composition; based on density or American Petroleum Institute (API) gravity; based on viscosity; based on carbon distribution; correlation index among others.
Petroleum and natural gas are classified as naturally occurring mixtures of hydrocarbons which can be separated into their original hydrocarbon constituents which have not been altered by any applied process.
The hydrocarbon constituents, separated from petroleum and natural gas, are the hydrocarbon constituents that existed in the reservoir. Naturally occurring hydrocarbons are major contributors to the composition of petroleum and natural gas.
Another classification of crude oil is based on the hydrocarbon chemical composition of crude oil. Petroleum is therefore classified as:
Paraffinic: these are crude oils that contain saturated hydrocarbons with straight or branched chains in large quantity but with less quantity of ring structure.
Naphthenic (Cycloparaffins):Crude oil with high ratio of saturated hydrocarbons containing one or more rings each are classified as naphthenic crude oil. The ring may have one or more paraffin side-chains. This structure is also known as alicyclic hydrocarbons.
Aromatics: These are crude oil with hydrocarbons containing large quantity of one or more aromatic nuclei such as benzene, naphthalene, and phenanthrene ring systems. The aromatic rings may be substituted with naphthalene rings or paraffin side-chains.
The table below gives a summary of chemical composition of the various classes of petroleum based on these three components:
Table: Chemical composition of the various classes of Petroleum
| Crude Oil | Paraffin | Naphthene | Aromatic | Wax | Asphalt | |
| Classification | (% | (%) | (%) | (%) | (%) | |
| Paraffinic | 46 – 61 | 22 – 32 | 12 – 25 | <10 | <6 | |
| Paraffinic– Naphthenic | 42 – 45 | 38 – 39 | 16 – 20 | <6 | <6 | |
| Naphthenic | 15 – 26 | 61 – 76 | 8 – 13 | 0 | <6 | |
| Aromatic | <8 | 57 – 78 | 20 – 25 | <0.5 | <20 | |
| Paraffinic– Naphthenic- Aromatic | 27 – 35 | 36 – 47 | 26 – 33 | <1 | <10 |
The use of density to classify crude oil is also well accepted. Density (specific gravity) is the principal, and the main specification of crude oil products used as an index of the proportion of gasoline and, particularly, kerosene in crude oil fraction.
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Crude oils of various types are also rated by specific gravity similar to gasoline and naphtha within certain limits of other properties.
The API gravity expresses the relative masses of crude oils. A low API gravity indicates a heavier crude oil, while a higher API gravity means a lighter crude oil. Specific gravities of crude oils roughly range from 0.82 for lighter crudes to over 1.0 for heavier crudes.
This is used to classify crude oil into three categories of paraffin, intermediate, or naphthene base crude oil. Investigation of crude oils from worldwide sources has shown that 85% fell into one of these three classes.
Crude oil fractions with API gravity >30.0 is paraffin based (specific gravity < 0.876) while API gravity in the range of 20.1–29.9 is intermediate between paraffin and naphthene and API gravity between <20 is naphthene based (specific gravity > 0.934).
Crude Oil Properties
Crude oil properties differs base on origin and the ratio of the different components in the mixture and influence the economic value. For instance, light crude oils yield more light and middle distillates like gasoline which are sold at higher prices making it more valuable.
Conversely, crudes containing high quantities of impurities (sulphur and nitrogen compounds) are less desirable because of their adverse effect on catalyst, reaction vessel and pipelines. This implies extra treating cost. Thus the properties of crude oil give the first-hand information about the crude and its value.
The following are some of the important crude oil properties determined by specific tests:
Density, Specific Gravity and API Gravity
The density of crude oil is the express of the mass of unit volume of the crude oil at a specific temperature. The property is determine by a specific gravity bottle and can be reported as API gravity as expressed above.
Salt Content
The salt content of crude oil is expressed in milligrams of sodium chloride per litre of the oil (or in pounds/barrel). It is an indicator of the amount of salt dissolved in water which is present in emulsified form with the crude oil.
A high salt content is not desirable in a crude oil because of serious corrosion problems associated with it in the refining process. Salt content that is high is also responsible for plugging heat exchangers and heater pipes.
Therefore, if salt content is higher than 10 lb. (NaCl)/1,000barrels of oil, the crude oil must be desalted.
Sulphur Content
Sulphur compounds can be present in crudes. Knowledge of its quantity is important because the amount of sulphur determines the type of treatment required in the refining processes.
Determination of sulphur content is carried out by weighing crude oil sample and burning it in air stream to oxidize it to sulphur dioxide and finally to sulphur trioxide.
The SO3 is dissolved in water to give H2SO4 and titrated with a standard alkali. Sulphur compounds are easily treated by hydrodesulphurization to produce hydrogen sulphide and the corresponding hydrocarbon.
PourPoint
The lowest temperature at which a crude oil or its product is observed to flow under the test conditions is the pour point of the oil. It is an indicator of the quantity of petroleum wax i.e. long-chain hydrocarbon found in the crude oil.
Paraffinic class of crude oil contains higher wax content than other crude types. The pour point of oil is important because the handling and transportation of crude oils and heavy fraction is difficult at temperatures below their pour points.
At such temperature, the oil will refuse to flow unless chemical additives (i.e. pour point depressants) are used to improve the flow properties of the fuel. The presence of long-chain n-paraffin of carbon 16–60 atoms will cause a near-ambient temperature precipitation and stop the flow of the oil.
Ash Content
Ash content is used to determine the amount of metallic constituents in a crude oil. Ash are inorganic components that cannot be volatilized at high temperature.
The CHNOS components of the crude oil can be oxidized during the ashing process in which there is complete burning the oil to give the stable metallic salts, metal oxides, and silicon oxide.
The residual ash could be further analysed for individual elements using atomic absorption spectroscopic techniques.
In summary, the components of crude oil are generally the same classes of compounds but their percentage composition vary from well to well or field to field. Ni/V ratio is used to investigate variations in the source materials and also variations in the conditions (geophysical) under which the petroleum was formed.
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Crude oil is generally classified based on the percentages of paraffin, nathphene and aromatic content. It can also be differentiated in the global oil market into sweet and sour crude based on the sulphur content.
To understand crude oil, determination of its properties like API gravity, salt content, sulphur content, ash content, pour point among others is very essential.