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Crude Oil: Petroleum pumped from a well is classified several ways

Liquid petroleum that is pumped out of an oil well is called “crude oil” or “crude.” Composed predominantly of carbon, crude oil contains approximately 84-87 percent carbon and 11-13 percent hydrogen. Crude oil also contains varying amounts of oxygen, sulfur, nitrogen, and helium.

The petroleum industry often names crude based on the oil’s geographical source–for example “West Texas Intermediate.” Crude oils are also classified based on physical characteristics and chemical composition using terms such as “sweet” or “sour,” “light” or “heavy.” Crudes vary in their price, usefulness as manufacturing feedstock, and impact on the environment.

 “Sweet” crude oil:

Crude oils with low sulfur content are classified as “sweet.” Those with a higher sulfur content are classified as “sour.” Sulfur content is generally considered an undesirable characteristic with respect to both processing and end-product quality. Therefore, sweet crudes are typically more desirable and valuable than sour crudes.

“Light” crude oil:

Crudes can be classified as “light” or “heavy,” a characteristic which refers to the oil’s relative density based on the American Petroleum Institute (API) Gravity. This measurement reflects how light or heavy a crude oil is compared to water. If an oil’s API Gravity is greater than 10, it is lighter than water and will float on it. If an oil’s API Gravity is less than 10, it is heavier than water and will sink.

Lighter crudes are easier and less expensive to produce. They generally have a higher percentage of light hydrocarbons that can be recovered with simple distillation at a refinery.

Heavy crudes can’t be produced, transported, and refined by conventional methods because they have high concentrations of sulfur and several metals, particularly nickel and vanadium. They have density approaching or even exceeding that of water. Heavy crude oils are also known as “tar sands” because of their high bitumen content.

 Crude oil’s economic value:

Generally, the less processing or refining a crude oil must undergo, the more valuable it is considered. Price differentials between crude oils typically reflect the ease of refining.

Crude oil can be refined to create products ranging from asphalt and gasoline to lighter fluids and natural gas, along with a variety of essential elements such as sulfur and nitrogen. Petroleum products are also key components in the manufacture of medicines, chemicals and plastics.

The lighter the oil, the more of the desirable, in-demand products it will produce through distillation at a range of temperatures. At the lowest distillation temperatures, products produced include liquid petroleum gases (LPG), naphtha, and so-called “straight run” gasoline. In the middle range of distillation temperatures, the refinery produces jet fuel, home heating oil and diesel fuel for transportation vehicles as well as construction and farm equipment.

At the highest distillation temperatures – over 1,000 degrees Fahrenheit – the heaviest products are produced, including residuum or residual fuel oil, which can be used for lubricants. To maximize output of more-desirable products, refineries commonly reprocess these heaviest products into lighter products.


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