Crude oil extracted from the ground is a mainly liquid mixture of hydrocarbons found in porous rocks beneath the Earth’s surface. It also contains substances like sulfur (1-3%), nitrogen (<1%), oxygen (<1%), and tiny amounts of metals and salts.
Crude oil varies significantly by region. For example, U.S. based crude is light and often referred to as sweet. It is great for manufacturing engine oils. Crude oils from other regions are usually heavier. For example, Venezuelan crude is heavy and well suited for producing asphalt and feedstock for refineries.
As noted in an edition of Stanford University’s Understand Energy Learning Hub, crude oils are classified by three main characteristics:
- API gravity is a measure of a crude oil’s density (how it flows), which was developed as a standard by the American Petroleum Institute.
- Light crude oil has high API gravity (generally >35 degrees) so it flows easily (like vegetable oil).
- Intermediate crude oil has medium API gravity (~25-35 degrees) so it doesn’t flow as easily as light crude oil does.
- Heavy crude oil has low API gravity (generally <25 degrees) and flows more like molasses or peanut butter. Sometimes it is upgraded prior to refining.
2. Sulfur content
- Sour is high sulfur crude oil.
- Sweet is low sulfur crude oil.
3. Geographical location of origin
Light, sweet crude oils are usually priced higher than heavy, sour crude oils. Light crudes are easier to refine into high demand products like gasoline and jet fuel because their density is closer to the density of those fuels. Sweet crudes are easier to refine into low-sulfur fuels that meet air quality standards. Proximity of crude oil production to major markets and transportation systems also impacts the price of crude oils.
Most refineries were built to accommodate a certain type of crude. It can be costly to retool to accommodate other types of crude, or at the very least, cause the refinery to operate in an inefficient manner. This video from the BBC explains: https://www.youtube.com/watch?v=ynsCt9jBbbo&t=113s