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Crude Oil

Crude Oil

Crude oil results from physical and chemical processes acting over many million years on the buried remains of plants and animals. Although crude oil is usually formed in fine-grained source rocks, it can migrate into more permeable reservoir rocks and large accumulations of petroleum, the oilfields, are accessed by drilling.

Crude oil results from physical and chemical processes acting over many million years on the buried remains of plants and animals. Although crude oil is usually formed in fine-grained source rocks, it can migrate into more permeable reservoir rocks and large accumulations of petroleum, the oilfields, are accessed by drilling.

Each oilfield produces a different crude oil which varies in chemical composition and physical properties.

Some crude oils, ‘crudes’, have a low sulphur content and flow easily.

Others may contain wax and flow only when heated.

Others contain very large amounts of very high molecular weight asphalt

Despite the wide range of hydrocarbons and other organic molecules found in crude oils, the main differences between crudes are not the types of molecules but rather the relative amounts of each type that occur in each crude oil source.

Components of Crude Oil

The components of crude oil can be classified into a few broad categories. Some of these components have properties desirable in a lubricant whilst others have properties which are detrimental.

1. Hydrocarbons:

Organic compounds composed exclusively of carbon and hydrogen) predominate in all crude oils and can be further subdivided into the following:

alkanes, known as paraffins, with saturated linear or branched-chain structures.

Linear Alkane

Branched Alkane

alkenes, known as olefins, unsaturated molecules, but comparatively rare in crude oils. Certain refining processes produce large amounts of alkenes by cracking or dehydrogenation.

Alkene

Alicyclics, known as naphthenes, are saturated cyclic structures based on five- and six-membered rings.

cyclohexane

Aromatics, cyclic structures with conjugated double bonds, mainly based on the six-membered benzene ring.

Aromatic

This is a simplified classification because many hydrocarbons can be combinations of these classes, e.g. alkyl-substituted cyclic or mixed poly-cyclics containing both aromatic and fully saturated rings.

2. Non-hydrocarbons:

Many organic compounds in crude oil incorporate other elements, sometimes within ring structures or as functional groups attached to a hydrocarbon structure.

Organo-metallic Compounds, Within the boiling range appropriate to lubricant base oils, almost all organo-sulphur and organo-nitrogen compounds are heterocyclic molecules

Organo-sulphur compund

Organo-nitrogen compound

The principal oxygen-containing molecules are carboxylic acids as either saturated aliphatic acids or cycloalkanoic acids (naphthenic acids).
Traces of phenols and furans may also occur.

3. High Molecular Weight Component:

There are very high molecular weight resins and asphaltenes which contain a variety of aromatic and heterocyclic structures.

Resins are the lower molecular weight, <1000 amu, species.

Asphaltenes result from linking together many other structures and have exceptionally high molecular weights.

Chemist Abdelrhman Sabry

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