The terms sweet and sour are a reference to the sulfur content of crude oil. Early prospectors would taste oil to determine its quality, with low sulfur oil actually tasting sweet. Crude is currently considered sweet if it contains less than 0.5% sulfur.
Sweet crude is easier to refine and safer to extract and transport than sour crude. Because sulfur is corrosive, light crude also causes less damage to refineries and thus results in lower maintenance costs over time. Due to all these factors, sweet crude commands up to a $15 dollar premium per barrel over sour.
Major locations where sweet crude is found include the Appalachian Basin in Eastern North America, Western Texas, the Bakken Formation of North Dakota and Saskatchewan, the North Sea of Europe, North Africa, Australia, and the Far East including Indonesia.
Sour crude oil will have greater than 0.5% sulfur and some of this will be in the form of hydrogen sulfide. Sour crude also contains more carbon dioxide. Most sulfur in crude is actually bonded to carbon atoms, nevertheless, high quantities of hydrogen sulfide in sour crude can pose serious health problems or even be fatal.
Hydrogen sulfide is famous for its “rotten egg” smell, which is only noticed at low concentrations. At moderate concentrations, hydrogen sulfide can cause respiratory and nerve damage. At high concentrations, it is instantly fatal. Exposure to high levels of hydrogen sulfide is thought to be in part responsible for Gulf War Syndrome, which is characterized by chronic fatigue, headaches, dizziness, memory problems, serious breathing problems, and even birth defects. Hydrogen sulfide is so much of a risk that sour crude has to be stabilized via removal of hydrogen sulfide before it can be transported by oil tankers.
Sour crude is more common in the Gulf of Mexico, Mexico, South America, and Canada. Crude produced by OPEC Member Nations also tends to be relatively sour, with an average sulfur content of 1.77%.