Crude Oil Classifications

Crude oil is classified by its non-hydrocarbon content (especially sulfur), its American Petroleum Institute (API) gravity, and pricing benchmarks. Crude oil containing low amounts of sulfur (<0.42 percent) and trace amounts of hydrogen sulfide and carbon dioxide is called sweet crude. Sweet crude oil is typically processed into gasoline. Sour crude oil has total sulfur amounts greater than 0.5 percent and higher hydrogen sulfide (>1 percent) and carbon dioxide concentrations. Because of the impurities in sour crude, it is often converted to heavy crude oil products, such as diesel and fuel oil.

API gravity is an indicator of how heavy crude oil is compared to water. Oil with an API > 10° is lighter than water, while an API < 10° is heavier than water and will sink. In general, light crude oils have low viscosity, low wax content, and low density/high API gravity (>30° API). Medium crude oil has API gravity between approximately 20° API and 30° API. Heavy crude oil is more viscous, and has a higher density/lower API gravity (10–20° API). Extra heavy crude oil (also called bitumen) has an API gravity less than 10° API.

Because a large proportion of light sweet crude oil can be directly processed into gasoline and other petroleum products, it typically commands the highest prices on commodity markets. An example of light sweet crude is West Texas Intermediate (WTI) Crude from the Texas Permian Basin. It has an API gravity of ~39.6° and sulfur content of ~0.24 percent. WTI crude is used as a benchmark for crude oil prices, along with other common benchmarks such as North Sea Brent, the Organization of the Petroleum Exporting Countries (OPEC) Reference Basket, and United Arab Emirates (UAE) - Dubai.

Natural Gas Categories

Natural gas is often categorized based on its liquid content. Dry natural gas is almost entirely methane and can be extracted from traditional reservoir rocks or from coal seams. It contains less than 0.1 gallon of liquid fractions per 1,000 cubic feet of produced gas.

Wet natural gas contains a larger proportion of natural gas liquids than dry gas. Natural gas liquids (NGLs) are fractions of natural gas that are liquid at surface conditions and are often separated from dry natural gas in processing facilities. NGLs with a low vapor pressure are called condensates, while NGLs with medium and high vapor pressures are called natural gasoline and liquefied petroleum gas, respectively. Examples of NGLs include propane, butane, isobutene, hexane, heptane, and pentane. Ethane is not typically considered an NGL because it needs to be refrigerated in order to maintain a liquid state. Wet gas typically sells at higher prices than dry gas.

Graph of density and sulfur content of selected crude oils
Modified from U.S. Energy Information Administration to include select Wyoming crude oils.WTI = West Texas Intermediate; LLS = Louisiana Light Sweet; FSU = Former Soviet Union; UAE = United Arab Emirates.


Rachel Toner,

Derek Lichtner,