Crude oil


Crude oil, liquid petroleum that is found accumulated in various porous rock formations in Earth’s crust and is extracted for burning as fuel or for processing into chemical products. A summary treatment of crude oil follows. For full treatment, see petroleum, petroleum production, and petroleum refining.

Crude oil is a mixture of comparatively volatile liquid hydrocarbons (compounds composed mainly of hydrogen and carbon), though it also contains some nitrogen, sulfur, and oxygen. Those elements form a large variety of complex molecular structures, some of which cannot be readily identified. Regardless of variations, however, almost all crude oil ranges from 82 to 87 percent carbon by weight and 12 to 15 percent hydrogen by weight.

Crude oils are customarily characterized by the type of hydrocarbon compound that is most prevalent in them: paraffins, naphthenes, and aromatics. Paraffins are the most common hydrocarbons in crude oil; certain liquid paraffins are the major constituents of gasoline (petrol) and are therefore highly valued. Naphthenes are an important part of all liquid refinery products, but they also form some of the heavy asphaltlike residues of refinery processes. Aromatics generally constitute only a small percentage of most crudes. The most common aromatic in crude oil is benzene, a popular building block in the petrochemical industry.

Because crude oil is a mixture of such widely varying constituents and proportions, its physical properties also vary widely. In appearance, for instance, it ranges from colourless to black. Possibly the most important physical property is specific gravity (i.e., the ratio of the weight of equal volumes of a crude oil and pure water at standard conditions). In laboratory measurement of specific gravity, it is customary to assign pure water a measurement of 1; substances lighter than water, such as crude oil, would receive measurements less than 1. The petroleum industry, however, uses the American Petroleum Institute (API) gravity scale, in which pure water has been arbitrarily assigned an API gravity of 10°. Liquids lighter than water, such as oil, have API gravities numerically greater than 10. On the basis of their API gravity, crude oils can be classified as heavy, medium, and light as follows:

Heavy: 10–20° API gravity

Medium: 20–25° API gravity

Light: above 25° API gravity

Crude oil also is categorized as “sweet” or “sour” depending on the level of sulfur, which occurs either as elemental sulfur or in compounds such as hydrogen sulfide. Sweet crudes have sulfur contents of 0.5 percent or less by weight, and sour crudes have sulfur contents of 1 percent or more by weight. Generally, the heavier the crude oil, the greater its sulfur content. Excess sulfur is removed from crude oil during refining, because sulfur oxides released into the atmosphere during combustion of oil are a major pollutant.

In the United States, the conventional practice for the petroleum industry is to measure capacity by volume and to use the English system of measurement. For this reason, crude oil in the United States is measured in barrels, each barrel containing 42 gallons of oil. Most other areas of the world define capacity by the weight of materials processed and record measurements in metric units; therefore, crude oil outside the United States is usually measured in metric tons. A barrel of API 30° light oil would weigh about 139 kg (306 pounds). Conversely, a metric ton of API 30° light oil would be equal to approximately 252 imperial gallons, or about 7.2 U.S. barrels.

Crude oil occurs underground, at various pressures depending on depth. It can contain considerable natural gas, kept in solution by the pressure. In addition, water often flows into an oil well along with liquid crude and gas. All these fluids are collected by surface equipment for separation. Clean crude oil is sent to storage at near atmospheric pressure, usually aboveground in cylindrical steel tanks that may be as large as 30 metres (100 feet) in diameter and 10 metres (33 feet) tall. Often crude oil must be transported from widely distributed production sites to treatment plants and refineries. Overland movement is largely through pipelines. Crude from more isolated wells is collected in tank trucks and taken to pipeline terminals; there is also some transport in specially constructed railroad cars. Overseas transport is conducted in specially designed tanker ships. Tanker capacities vary from less than 100,000 barrels to more than 3,000,000 barrels.

The primary destination of crude oil is a refinery. There any combination of three basic functions is carried out: (1) separating the many types of hydrocarbon present in crude oils into fractions of more closely related properties, (2) chemically converting the separated hydrocarbons into more desirable reaction products, and (3) purifying the products of unwanted elements and compounds. The main process for separating the hydrocarbon components of crude oil is fractional distillation. Crude oil fractions separated by distillation are passed on for subsequent processing into numerous products, ranging from gasoline and diesel fuel to heating oil to asphalt. Given the pattern of modern demand (which tends to be highest for transportation fuels such as gasoline), the market value of a crude oil generally rises with increasing yields of light products.

Crude Oil Qualities in Iraq

Crude oil found in Iraq varies significantly in quality, with API gravities generally ranging from 22° (heavy) to 35° (medium - light).[1] Over 70% of national oil reserves are below 28° API[2] and the International Energy Agency (IEA) predicted in its 2012 report on Iraq that future production is likely to include a larger share of heavier crudes.[3] However some of the crudes produced at the Taq Taq field in the norther semi-autonomous Kurdistan region are as light as 48° API, dubbed by Reuters as "champagne crude".[4]

Reuters reported in November 2012 that Iraq was struggling to find buyers for its 2013 oil output due to complaints from refiners in Asia, Europe and the US over high prices and variable quality.[5]

Export Blends

The main export crudes come from Rumaila and Kirkuk, the two largest active fields. The two blends used for export are the Basra Light blend, transported by tanker from the south, and the Kirkuk blend, by pipeline to the north.[6] In terms of quality, the Basra Light blend is in the middle of the market, close to the global average density of close to 32.5° API.[7]

The API gravity (how 'heavy' the oil is) and the sulphur content (how 'sour' the oil is) for each blend is shown in the table below.

Due to the large number of fields and differing grades of crude (particularly in the South), as well as the limited sea outlet and export routes, the crude blending process in Iraq can be problematic.[10]and according to the IEA, the country "has to offer discounts to compensate for the specification of the delivered oil being heavier than the contractual figures, as a result of the heavier crudes and heavy fuel oil being blended into the export stream."[11]

A former official at the OPEC Secretariat comments that Iraq lacks a proper blending system to insure a relatively stable quality of its crude oil export streams.[12] However the IEA report also notes that much of future demand for Iraqi crudes is to come from Asia, where large, modern refineries are equipped to deal with processing a range of specifications.[13]

Related Links

Encyclopedia Britannica

Open Oil


1.↑ "Iraqi Crude Heavier Due to Fuel Blending: Traders" Iraq Energy, 1 June 2011.

2.↑ "Iraq oil: The crude oil quality dilemma" Gulf News, 11 November 2012.

3.↑ "Iraq Energy Outlook" IEA, 6 September 2012.

4.↑ "Kurdistan Taq Taq oil exports rise ahead of Sept deadline" Reuters, 9 October 2012.

5.↑ "Light crude surplus spins world oil trade compass" Reuters, 16 November 2012.

6.↑ "Iraq oil: The crude oil quality dilemma" Gulf News', 11 November 2012.

7.↑ "Iraq oil: The crude oil quality dilemma" Gulf News', 11 November 2012.

8.↑ "Iraqi Crude Heavier Due to Fuel Blending: Traders" Iraq Energy, 1 June 2011.

9.↑ "Iraqi Crude Heavier Due to Fuel Blending: Traders" Iraq Energy, 1 June 2011.

10.↑ "Iraq oil: The crude oil quality dilemma" Gulf News', 11 November 2012.

11.↑ "Iraq Energy Outlook" IEA, 9 October 2012.

12.↑ "Iraq oil: The crude oil quality dilemma" Gulf News', 11 November 2012.

13.↑ "Iraq Energy Outlook" IEA, 9 October 2012.

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