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According to modern view, the oil has been produced as a result of bacterial decomposition of the remains of animals and plants (zooplankton, algae, etc.) under high pressure which got buried under the sea, millions of years ago. The hydrocarbons formed by the decay of microorganisms rise through porous rocks until they are trapped by impervious rocks. If the region is uplifted during the passage of time, the oil trap will be found to lie under land. Oil in these deposits generally floats over salt water. The oil in the petroleum field is generally covered with a gaseous mixture known as natural gas. Natural gas mainly contains methane, ethane, propane and butane.

Specific Objectives

After studying this unit the students will be able to

  • identity the chemical elements and compounds found in crude oil
  • outline the formation of crude oil from biological sources
  • demonstrate understanding of the classification of crude oil
  • describe how crude oil is extracted from an oil well
  • describe the fractional distillation of crude oil
  • explain cracking and reforming of organic compounds
  • outline the uses of the fractions obtained from crude oil distillation
  • outline the sources and uses of petrochemicals
  • explain octane number and its importance to the petroleum industry.






On the basis of sulphur content crude oil is classified as sweet crude oil or sour crude oil.


  1. Sweet Crude Oil. Petroleum is considered sweet if it contains less than 0.5% sulphur. Sweet crude oil contains small amounts of hydrogen sulphide and carbon dioxide. The term sweet originates because the low level of sulphur provides the oil with a mildly sweet taste and pleasant smell.
  2. Sour Crude Oil. Petroleum is considered sour if it contains more than 0.5% sulphur.




On the basis of wax content and viscosity, crude oil is classified as light or heavy crude oil.


  1. Light Crude Oil. It is the crude oil with low wax content. It is less viscous.
  2. Heavy Crude Oil. It is the crude oil with higher wax content, It is more viscous and therefore is more difficult to transport and pump.


Do You Know?

  • Sweet crude oil commands a higher price-than sour oil because it has. fewer environmental problems and requires less refining
  • Light crude oil is more desirable than heavy crude oil because it produces higher yields of gasoline, kerosene and high quality diesel.




The American Petroleum Institute gravity or API gravity is a measure of how heavy or light a petroleum liquid is compared to water. If API gravity of petroleum is greater than 10, is lighter and floats on water, if less than 10, it is heavier and sinks. API gravity is used to compare the relative densities of petroleum liquids.


For example, if one petroleum liquid floats on another and is therefore less dense. It has a greater API gravity.


Crude oil is classified as light, medium or heavy according to its measured API gravity

  1. Light Oil has an APl gravity higher than 31.1o API
  2. Medium Oil has an API gravity between 22.3o API and 31.1 °API
  3. Heavy Oil has an API gravity below 22.3o API


Do You Know?

  • Crude oil with an API gravity between 40° and 45° commands the highest price.
  • Crude oil with API gravity less than 10° API is referred to as extra heavy oil or bitumen.



The oil industry also classifies crude oil by the geographic location of its origin. Some classes of crude on this basis are given below:


1.West Texas Intermediate (WTI) is the reference used for North American Crude oil. It is very high quality sweet light oil.

2.Brent Crude or Brent Blend is the benchmark used for crude oil from North Sea. Brent Blend is a light crude oil though not as light as WTI.

3.Dubai-Oman is the benchmark used for the crude oil from Middle East

4.OPEC Basket is the reference used for crude oils from various OPEC (The Organisation of the Petroleum Exporting Countries) countries.


Thus, crude oils from different sources have different qualities and hence command different prices.

Now, can you answer why there are differences in prices, for the same quantity of crude oil from different sources.