20.2 Crude Oil

  • Proportions of elements in petroleum (weight%):

    Carbon 83–87
    Hydrogen 10–14
    Nitrogen 0.1–2.0
    Oxygen 0.05–1.5
    Sulfur 0.05–6.0
    Metals (Ni and V) \(<1000\) ppm


    • Sulfur content: Oil with less than 1 wt.% sulfur is referred to as low sulfur or sweet, and that with more than 1 wt.% sulfur is referred to as high sulfur or sour.

    • Oxygen content: A phenomenally high oxygen content indicates that the oil has suffered prolonged exposure to the atmosphere.

    • Nitrogen content: In general, the more asphaltic (high naphthene content) the oil, the higher is its nitrogen content. Nitrogen compounds are more stable than sulfur compounds and therefore are harder to remove. They can be responsible for the poisoning of a cracking catalyst, and they also contribute to gum formation in finished products.

    • Because of the narrow range of carbon and hydrogen content, it is not possible to classify petroleum on the basis of carbon content as coal is classified; carbon contents of coal can vary from as low as 75% in lignite to 95% in anthracite.

  • Kerogen is the naturally occurring, solid, insoluble organic matter that occurs in source-rocks and can yield oil upon heating.

  • The hydrocarbons present in crude petroleum are classified into three general types: paraffins, naphthenes, and aromatics. Due to their high reactivity, olefins are not common in natural crude oil, and are formed during the processing of petroleum. Cycloparaffin hydrocarbons in which all of the available bonds of the carbon atoms are saturated with hydrogen—are called naphthenes.

  • API Gravity: The density of petroleum oils is expressed in terms of API gravity (or \(^{\circ}\)API) rather than specific gravity (SG). Increase in API gravity corresponds to a decrease in specific gravity. Most crudes fall in the 20 to 45\(^\circ\)API range.

    \[{}^{\circ}\text{API} = \frac{141.5}{\text{SG}} - 131.5\]