Main-Product

General Properties and Composition

Crude oil is a mixture of liquid hydrocarbons and their other natural forms directly extracted from underground geological formations (or reservoirs). It is a fossil fuel, which means it is formed from dead organisms that are buried under intense heat and pressure. It is usually a flowing or semi-flowing viscous liquid.

Both the properties and appearance of crude oil vary with the region from which it is produced. In terms of color, most crude varieties are black, while some are dark grey, dark green, dark brown, or even reddish brown, light yellow, or colorless; in terms of specific gravity, most varieties fall between 0.8 and 0.98. Crude oil also tends to be smelly due to the sulfur compounds it contains.

Crude oil is mainly composed of carbon, hydrogen, sulfur, nitrogen, oxygen, and trace elements. Carbon and hydrogen are the main constituents, accounting for approximately 96–99% of crude oil by weight, with the rest accounting for no more than 1–4%. All these elements exist in crude oil in the form of organic compounds, most of which are hydrocarbons including alkanes, cycloalkanes, aromatics, and hydrocarbon mixtures having the molecular structures of all three. Crude oil doesn’t normally contain alkenes or alkynes, but the former does exist in some products from the second stage of refining. Aside from hydrocarbons, crude oil also has a considerable amount of non-
hydrocarbon compounds, mainly including compounds containing sulfur, oxygen, nitrogen, as well as gelatinous and asphaltic substances (resins and asphaltenes), which altogether representing 10–20% of crude oil by mass.

There are over one hundred different crude oils traded on the market today. These oils are typically labeled by the region they come from, and they have a specific chemical makeup.

Main Performance Indicators

The main performance indicators of crude oil and petroleum products include, among others, density (or weight), viscosity, solidifying point, asphaltene and resin content, sulfur content (or sweetness), wax content, wax appearance point (cloud point), water content, total acid number (TAN), flash point, specific heat, and explosive limit. For crude oil, the physical properties are key indicators in determining its quality (and that of the resulting products) and controlling the refining process.

• Weight of Crude Oil: Light Oil vs. Heavy Oil
   
  • Heavy oil evaporates slowly and contains material that will be used to make heavy products like asphalt.
  • Light oil requires less processing and produces a greater percentage of gasoline and diesel than heavy oil.

• Crude Oil Density
  
  The density of crude oil refers to its mass per unit volume. In general, lower density crude oil yields a higher ratio of light oil. Because rising temperature increases a petroleum product’s volume and thus reduces its density, density is only meaningful alongside a temperature. China’s national standard (GB/T 1884) prescribes that the density at 20 °C, represented by ρ₂₀, is to be the standard density of petroleum and liquid petroleum products in China.
   
• Specific Gravity
  
  The specific gravity of a petroleum product refers to the ratio of its density to the density of water at a specified temperature, and is generally denoted as d (4,t) for petroleum product at t °C. In China and Eastern Europe, the commonly used 20°C specific gravity is d_{(4°C, 20°C)}, whereas in America and other European countries it’s d_{(60°F,60°F)}, i.e., the ratio of the density of petroleum product at 60 °F to that of water at 60 °F (approx. 15.6 °C).
  
  The standard unit of measurement for oil weight is API Gravity at 60°F  ("API Gravity" for short), a scale was created by the American Petroleum Institute to measure the density of oil. API gravity is the opposite of density, in that a larger API gravity means a lower density. At present, API gravity is one of the primary factors in the global pricing of crude oil. Higher API gravity means a lighter crude oil and commands a higher price:

• Viscosity
  
  The viscosity of crude oil can be expressed and measured in a number of ways which often vary across countries. China uses kinematic viscosity and the Engler scale; countries including the U.S. and Great Britain use the Saybolt and Redwood scales; most countries in Western Europe, including Germany, adopt the same standards as China. The International Standardization Organization (ISO) has named kinematic viscosity to be the universal standard, a brief overview for which is given below. The kinematic viscosity of crude oil refers to the ratio of its dynamic viscosity to its density. Dynamic viscosity is measured in pascal second (Pa * s) in the SI system and poise (P) or centipoise (cP) in the CGS system, interconvertible as follows:
   

  1 Pa * s = 10 P = 103 cP

  
  Kinematic viscosity, therefore, has a unit of m²/s or mm²/s in SI and stokes (St) or centistokes (cSt, or 1/100 St) in CGS. For example, “180 cSt fuel oil” means fuel oil with a kinematic viscosity of 180 cSt. The two systems of units are interconvertible as follows:
   

  1 m2/s = 106 mm2/s = 106 cSt

  
  Viscosity measures the resistance of crude oil to flow, and will decrease with rising temperature. Viscous, high pour-point crude oil and gas oil should be heated during shipment to maintain fluidity.
   
• Sweetness of Crude Oil
  
  What makes a particular crude sweet or sour is the amount of sulfur it contains. Sweet crude has very low levels of sulfur, well under 1%. Sour crude has as much as 1-2% of sulfur. Midstream companies and refiners that transport, store, and process sour oil know they need extra treating capabilities to take out the sulfur and sweeten the product. HIGH Sulfur Level such as  to 2% correspond to Sour Oil, whereas LOW Sulfur Level below 1% correspond to Sweet Oil. Sulfur compounds negatively affect the refining process and the application of petroleum products; for instance, they corrode metallic equipment and pipelines, cause catalyst poisoning, and lower product quality. Sulfur Oxides (SO₂, SO₃), and other pollutants from the combustion of sulfur-containing fuel can cause serious harm to the environment. Hence, it is essential to limit the sulfur content in petroleum products; in particular, crude oil should undergo desulfurization before further processing.
   
• TAN Count of Crude Oil
  
  TAN stands for “Total Acid Number.” The TAN count of oil is a measure of how corrosive it is. If a crude has a high TAN number such as 3.0 mg KOH/g, producers must use more robust metallurgy than standard so their processes can handle that corrosivity and keep the crude in the pipe, compared with low TAN count such as 0.01 mg KOH/g that has very corrosivity.
   
• Low Temperature Performance
  
  A petroleum product’s low temperature performance is a vital quality indicator that directly affects how the product should be shipped, stored, and used. This performance is measured by a number of metrics, including cloud point, crystallization point, freezing point, solidifying point, pour point, and cold filter plugging point, with solidifying point and pour point being the most important. Solidifying point refers to the highest temperature that a petroleum product under prescribed thermal and shearing conditions can maintain a flowing surface when cooled. Pour point refers to the lowest temperature that a petroleum product can maintain its fluidity under prescribed laboratory conditions. The solidifying point of crude oil ranges from -50 °C to 35 °C depending on its composition. In particular, crude oil with a high proportion of light components has a lower solidifying point; conversely, crude oil with a high proportion of heavy components (especially paraffin) has a higher solidifying point.
   
• Combustion Performance
  
  Most petroleum products are flammable and explosive. Indicators like flash point, fire point, and auto-ignition temperature define the level of fire hazard – with lower values indicate a more easily combustible product – and are essential for ensuring personal and property safety during the storage and shipment of crude oil and petroleum products.
   
• Solubility
  
  Crude oil is insoluble in water but can be combined with it to form an emulsion; crude oil is soluble in organic solvents like benzene, essence, ether, chloroform
  and carbon tetrachloride, and partially soluble in alcohol.

Crude Oil Assay

Crude Oil Assay consists of a compilation of data on properties and composition of Crude Oils. The Assay provides critical information on the suitability of Crude Oil for a particular Refinery and estimating the desired Product yields and quality. It also indicates how extensively a given Crude Oil should be treated in a Refinery to produce Fuels that are in compliance with environmental regulations. A typical Crude Assay should include the following major specifications:^[2]

• API Gravity
• Total Sulfur (% wt)
• Pour Point (°C)
• Viscosity @ 20°C (cSt)
• Viscosity @ 40°C (cSt)
• Nickel (ppm)
• Vanadium (ppm)
• Total Nitrogen (ppm)
• Total Acid Number (mgKOH/g)
• Distillation Data
• Characterization factor K_UOP, K_W

A randomly selected, typical Crude Assay is shown in Table 1. 

Table 1: Bakken Crude Summary Report (Crude Assay)^[3]

Crude Oil Fractions

As a complex, multi-component substance, crude oil has a very wide boiling range from room temperature to above 500 °C. While each component has its own characteristics, it is unnecessary to break down crude oil into individual components to derive useful petroleum products. In general, simply separating crude oil by fractional distillation should be enough for research and processing purposes. The term “fractional distillation” means “cutting” crude oil into “fractions” by the different boiling points of its components. Such fractions are often referred to as gasoline, kerosene, diesel, lubricating oil, or some other petroleum products, but they are not petroleum products in the strict sense of the term because the latter must meet certain quality requirements, while the former is just intermediate or semi-finished products that require further
processing before becoming true petroleum products.

More details on crude oil fraction percentages are provided in the description of the Atmospherique Distillation process.

References

1. Crude Oil Futures Trading Handbook, 2024 Edition, Shaghai Internation Energy Exchange.
2. Kimray Inc.^®, Types of Crude Oil: Heavy vs Light, Sweet vs Sour, and TAN count
3. PennState College of Earth and Mineral Sciences, FSC 432: Petroleum Processing, Crude Assay
4. equinor, Crude oil assays