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Alkanes: Sources and Relevance

Alkanes: Formation and Sources of Crude Oil

Alkanes are found in nature mainly in natural gas and crude oil from which they are produced on a technical scale in large amounts by distillation and extraction. Natural gas mainly consists of methane, ethane and small amounts of propane. Propane is the main component of liquified gas which, compressed in steel bottles, is being used as heating fuel. Crude oil is mainly composed of alkanes and cycloalkanes which have been generated by decomposition of animal and plant matter. Coal is an additional source of raw material. It consists of a complex mixture of mostly high-molecular alicyclic, heterocyclic, saturated, unsaturated, and aromatic hydrocarbons formed from the original residues of plant matter.

Structure example for a macromolecule that might occur in soft coal

Exact knowledge about the formation of crude oil does not exist. However, current hypotheses based on the assumption that crude oil originated from animal and plant organisms have been corraborated by strong evidence. According to these hypotheses, burial of dead organisms in ocean sediments was followed first by bacterial anaerobic decomposition and subsequently by thermal decomposition under high pressure and high temperatures catalyzed by mineral catalysts. The fact that specific optically active compounds are found is an indication that preferably carbohydrates, proteins and fats were converted. Therefore, it must be concluded that crude oil has been formed from organic matter. Furthermore, it was found that chemical fossils show an age-dependent composition. Finally, the analysis of crude oil as shown below points to an organic origin.

  • Carbon: 85 - 90%
  • Hydrogen: 10 - 14%
  • Oxygen: 0 - 1,5%
  • Sulfur: 0,1 - 3% (very seldom to 7%)
  • As well as traces of nitrogen, chlorine, iodine, sodium, and potassium 0,001–0,05%

Crude oil or petroleum is not a pure compound but a mixture of hundreds of compounds of which hydrocarbons represent the majority.

Crude oil is mostly found in sedimentary rocks, such as limestone, sandstone and clay, which are part of ocean sediments. Single drops of crude oil agglomerate around the parent rocks, mostly layers of clay sediments. Because of its exceptional high capillary properties, crude oil migrates upwards through adjacent rock layers until being stopped by impermeable rock formations above. Oil reservoirs such formed represent 80% of total crude oil deposits. The areas of these deposits are called secondary deposits.

The oldest reservoir discovered up to now was formed about 2 billion years ago concurring with the existence of primitive life forms. However, the most important reservoirs were only formed 500 to 100 million years ago. Additional findings can also be dated to the Triassic, Jurassic and Cretaceous periods.

Based on 1999 data, the three most important oil producing countries and national organisations are:

  • The former UDSSR with 638 million tons
  • USA with 455 millionen tons
  • Organization of the Petroleum Exporting Countries (OPEC) with 1,415 million tons

On the other hand, production in Germany with only 4 million tons does not even cover the country's own needs.

The highest producing oil field in Germany, "Mittelplate", is located at the southern border of the National Park "Schleswig-Holsteinisches Wattenmeer" yielding 2.8 million tons (1996).

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