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Crude Oil Basics

Petroleum pumped from a well is classified several ways


Silhouette Of Pump Jack With The Orange Glow Of The Sun Rising On The Horizon
Michael Interisano/Design Pics/Getty Images

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.

What is “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.

What makes a crude oil “light”?

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. Heavy crudes have density approaching or even exceeding that of water. Heavy crude oils are also known as “tar sands” because of their high bitumen content.

With simple distillation, these dense, heavier crude oils produce a greater share of lower-valued products. Heavy crudes require extra refining to produce more valuable and in-demand products.

Petroleum in a nutshell

What determines crude oil’s relative 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.

Simple distillation – first-level refinement – of different crude oils produces different results. For example, the U.S. benchmark crude oil, West Texas Intermediate (WTI), has a relatively high natural yield of desirable end-products, including gasoline. But the process also yields about one third “residuum,” a residual by-product that must be reprocessed or sold at a discount. In contrast, simple distillation of Saudi Arabia's Arabian Light, the historical benchmark crude, yields almost half "residuum." This difference gives WTI a higher premium.

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.

Are some crude oils more toxic than others?

“Toxicity” refers to how harmful an oil might be to humans and other living organisms, as well as to land and water. Generally, the lighter the oil the more toxic it is considered.

Because of the constant potential of spills, the Environmental Protection Agency has classified crude oils in four categories that reflect how the oils would behave in a spill and its aftermath:

Class A – Because they are light and highly liquid, these clear and volatile oils can spread quickly on impervious surfaces and on water. Their odor is strong and they evaporate quickly, emitting volatiles. Usually flammable, these oils also penetrate porous surfaces such as dirt and sand and may remain in areas into which it has seeped. Humans, fish, and other biota face danger of toxicity to Class A oils. These high quality light crudes and the products produced from them are in this class.

Class B – Considered less toxic than Class A, these oils are generally non-sticky but feel waxy or oily. The warmer it gets, the more likely Class B oils can be to soak into surfaces and they can be hard to remove. When volatile components of Class B oils evaporate, the result can be a Class C or D residue. Class B includes medium to heavy oils.

Class C – These heavy, tarry oils (which include residual fuel oils and medium to heavy crudes) are slow to penetrate into porous solids and are not highly toxic. However, Class C oils are difficult to flush away with water and can sink in water, so they can smother or drown wildlife.

Class D – Non-fluid, thick oils are comparatively non-toxic and don’t seep into porous surfaces. Mostly black or dark brown, Class D oils tend to dissolve and cover surfaces when they get hot, which makes cleanup more harder. Heavy crude oils, such as the bitumen found in tar sands, fall into this class.

How do oil spills damage the environment?

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