Petroleum Fluids

Chemical classification of petroleum fluids

  • Petroleum fluids are complex multi-component mixtures.
  • The chemical constituents of petroleum may be classified broadly as belonging either to the C6- or the C6+ fraction.
  • The light end, or C6- fraction, of petroleum fluids is composed of well-defined pure hydrocarbon components with carbon numbers up to 5 and the light gases nitrogen (N2), carbon dioxide (CO2), and hydrogen sulfide (H2S).
  • The hydrocarbons in the light end primarily are straight-chain normal alkanes (n-alkanes) and their branched isomers (i-alkanes).
  • The heavy end, or C6+ fraction, consists of all the components with carbon numbers of 6 or greater.

Classification of petroleum constituents

A classification system and nomenclature commonly used in the petroleum industry describes components as belonging to the paraffinic (P), naphthenic (N), or aromatic (A) fractions. These are often referred to jointly as PNA.


This class includes n-alkanes and i-alkanes that consist of chains of hydrocarbon segments (-CH2-, -CH3) connected by single bonds. Methane (CH4) is the simplest paraffin and the most common compound in petroleum reservoir fluids. The majority of components present in solid wax deposits are high-molecular-weight paraffins.


This class includes the cycloalkanes, which are hydrocarbons similar to paraffins but contain one or more cyclic structures. The elements of the cyclic structures are joined by single bonds. Naphthenes make up a large part of microcrystalline waxes.


This class includes all compounds that contain one or more ring structures similar to benzene (C6H6). The carbon atoms in the ring structure are connected by six identical bonds that are intermediate between single and double bonds, which are referred to as:

  • Hybrid bonds
  • Aromatic double bonds
  • Benzene bonds

Resins and asphaltenes

Resins and asphaltenes primarily are a subclass of the aromatics, although some resins may contain only naphthenic rings. They are large molecules consisting primarily of hydrogen and carbon, with one to three sulfur, oxygen, or nitrogen atoms per molecule. The basic structure is composed of rings, mainly aromatics, with three to ten or more rings per molecule.

SARA classification of petroleum constituents

The components of the heavy fraction of a petroleum fluid can be separated into four groups: saturates, aromatics, resins, and asphaltenes (SARA).

  • Saturates include all hydrocarbon components with saturated (single-bonded) carbon atoms. These are the n-alkanes, i-alkanes, and cycloalkanes (naphthenes).
  • Aromatics include benzene and all the derivatives composed of one or more benzene rings.
  • Resins are components with a highly polar end group and long alkane tails. The polar end group is composed of aromatic and naphthenic rings and often contains heteroatoms such as oxygen, sulfur, and nitrogen. Pure resins are heavy liquids or sticky solids.
  • Asphaltenes are large highly polar components made up of condensed aromatic and naphthenic rings, which also contain heteroatoms. Pure asphaltenes are black, nonvolatile powders.

The experimental method used to determine the weight fractions of these groups is called SARA analysis.

Asphaltene characteristics

Nature of asphaltenes

  • Asphaltenes are a solubility class that is soluble in light aromatics such as benzene and toluene but is insoluble in lighter paraffins. 
  • They normally are classified by the particular paraffin used to precipitate them from crude (e.g., n-pentane or n-heptane). 
  • Mitchell and Speight showed that different alkane solvents yield different amounts of precipitates. 
  • Speight showed dependence of the aromaticity and molecular weight of asphaltene on the precipitating solvent.
  • They also indicated that the amounts and natures of asphaltenes precipitated with n-heptane or heavier alkanes are very similar.
  • Speight, Long and Trowbridge provides a summary of standard analytical methods for asphaltene separation with either n-pentane or n-heptane.

Asphaltenes and waxes

  • Deposition of the high-molecular-weight components of petroleum fluids as solid precipitates in surface facilities, pipelines, downhole tubulars, and within the reservoir are well-recognized production problems.
  • Depending on the reservoir fluid and the type of recovery process, the deposited solid may consist of:
    • Asphaltenes
    • Waxes
    • A mixture of these materials
  • The deposits also can contain resins, crude oil, fines, scales, and water.
  • Asphaltenes and waxes are a general category of solids and, thus, cover a wide range of materials.
  • Understanding the fundamental characteristics that define the nature of asphaltenes and waxes is valuable in reducing or avoiding the production impacts of their deposition.

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