Composition of Crude Oil

Crude Oil

Crude Oil
Crude oil is a natural multicomponent mixture. Its major part is composed of hydrocarbons (alkanes, naphthenes, and aromatics). Their content in oils ranges between 30% and 100%. Most important among the non-hydrocarbon components are resins and asphaltenes. The other non-hydrocarbon compounds are metal–porphyrin complexes and trace elements; their content is usually low. Some compounds in oils lost structural features of the parent organic matter, whereas some other molecules preserved these features. They are called ‘‘relic hydrocarbons’’ or ‘‘chemical fossils’’.

Alkane hydrocarbons (C5 –C40) include normal and branched molecules (isoprenoids). Carbon number distribution in the normal alkanes reflects the composition of the original organic matter. For example, lipids of the continental biomass are dominated by normal C25 –C33 alkanes that consequently are inherited by the oil. The pristane/phytane ratio is used as a genetic criterion for the isoprenoids. The pristane is associated with the continental deposits, whereas the phytane is associated with the marine deposits. 

Cyclic paraffins (naphthenes) include monocyclic (5–6 carbon atoms) as well as polycyclic molecules. The latter molecules may contain 1–6 rings. This feature was probably inherited from the parent organic matter (naphthene index). But most polycyclic naphthenes (such as steranes) were not present in the parent organic matter and have been formed during catagenesis. 
Arenes (aromatic hydrocarbons) are usually not as important as the other classes of hydrocarbons in crude oils. Aromatic compounds may include exclusively aromatic rings, or may contain complex structures with naphthalene rings. Some arenes are directly related to the parent organic matter. 
There are cyclic changes in the chemical properties of oils (contents of paraffin, asphaltenes, resins, and sulphur) with geologic age of rocks. This cyclicity is controlled by the cyclicity of ocean transgressions and processes of formation and development of paleo-oceans in the geological history of the Earth. 

Recently developed equipment and techniques drastically increased the information on oil composition. The researchers are now able to determine not only the group hydrocarbon composition, but also the composition of individual hydrocarbons and their structure. The new techniques include gas and liquid chromatography, spectral and isotope methods, and nuclear magnetic and paramagnetic resonances. Among the new highly sensitive equipment are chromatographs, chromato-mass spectrometers, and infrared, ultraviolet, quasi-linear, and isotope spectrometers. 

A heightened interest in the molecular- and atomic-level information on the oil composition was caused by two factors: technological and geochemical. Petroleum hydrocarbons currently serve as a source of wide spectrum of synthetic substances used for the manufacturing of various goods in food industry and other industries. This required detailed studies of the composition of individual hydrocarbons. The current technology provides an opportunity to obtain information on the detailed composition and structure of hydrocarbons found in the high-boiling oil fractions. Such information covers carbon atom distribution in the paraffin chains and in the naphthene and aromatic rings. Lately, this information also became insufficient. 

The emergence of such analytical techniques as the gas–liquid chromatography and chromato-mass spectrometry enabled scientists to 
  • Obtain new information on the composition and structure of petroleum hydrocarbons, 
  • Study in detail their homological series, and 
  • Determine the distribution patterns of normal and branched alkanes, methylalkanes, and isoprenoid alkanes in oils. 
In studying naphthenes, new techniques led to the elucidation of the proportions of mono-, bi-, tri-, and tetracyclic naphthenes, steranes and tri-terpanes (hopanes). Detailed studies of aromatic hydrocarbons in crude oils (using various techniques including spectral) resulted in the establishment of the presence and proportions of not only mono-, bi-, and tricyclic, but also polycyclic (4–6 cycles) hydrocarbons that were almost impossible to identify earlier. The latter include hydrocarbons such as perylene, 1,12-benzoperylene, 3,4-benzopyrene and their homologes. 

Nuclear-magnetic and paramagnetic resonance techniques developed in the 1950s enabled to study the properties of nuclei in different states. This is important in studying the free radicals (kinetically independent), atoms and atom groups, and chain reactions (polymerization, pyrolysis) in biochemical processes, in which the free radicals actively participate. 

A new approach in studying the crude oil hydrocarbons involves the stereochemistry of saturated aliphatic and alicyclic hydrocarbons. Stereochemical studies of the normal and branched alkanes and mono-, bi-, tri-, and tetracyclic hydrocarbons (including hopanes) are becoming more important in geochemical studies. It was shown that the transformations (aging) of biomolecules in the Earth’s crust is closely related to the changes in their stereochemistry. 

There is an increase in the trace element studies. Contents of the trace elements in crude oils vary significantly. Most of the iron series elements are found in crude oils in amounts below the clarke amounts (sedimentary rock clarke). Some elements (zinc, nickel, copper, arsenic, and silver) are found in near-clarke amounts, and four elements (vanadium, molybdenum, bromine, and mercury) are present in the amounts an order of magnitude above the clarke. This offers an opportunity of their recovery directly from crude oils. The recovery of trace elements from crude oils is technically complex and had not been commonly used, although scientific experimentation is in progress. 

On the basis of the extensive knowledge of composition and structure for all classes of hydrocarbons, the presence of bio-markers directly related to the parent biomass had been established. Genetic relationship of crude oils and parent organic matter, genetic uniformity (or non-uniformity) of oils in different stratigraphic sequences have been established. Transformations of the specific hydrocarbons within the catagenetic, weathering etc. zones had been elucidated. 
This information is important in the petroleum exploration as it enables to 
  • Forecast the type and composition of hydrocarbon fluids. 
  • Identify the potential cross-flow zones. 
  • Determine the paths of lateral and vertical migration.
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