It is common to find crude oil containing some impurities.
А particular crude sweet or sour is the amount of sulfur it contains.
Sweet crude has very low levels of sulfur, well under 1%.
Sour crude has as much as 1-2% of sulfur.
The presence of sulfur makes oil more difficult and costly to refine, causing sour crude to be viewed as a less desirable form of crude oil.
Sweet crude oil commands a higher price on international commodity markets.
Sweet crude is defined by the New York Mercantile Exchange (NYMEX) as petroleum with sulfur levels below 0,42%.
When the total sulfur level in the oil is more than 0.5% the oil is called sour.
The more costly sour crude less efficient as a source of energy production, decreasing its demand from commodity investors.
In an effort to reduce their total processing costs, sour crude producers often seek to refine sour crude into heavy oil products such as diesel and fuel oil (as opposed to gasoline).
Midstream companies and refiners that transport, store, and process sour oil know they need extra treating capabilities to take out the sulfur and sweeten the product.The impurities need to be removed before this lower-quality crude can be refined into petrol, thereby increasing the cost of processing.
This results in a higher-priced gasoline than that made from sweet crude oil.
In order to prepare sour crude for sale on commodities markets, oil refineries must use a process known as cracking to separate the dozens of hydrocarbon compounds contained in the oil into separate chemical units.
The refineries must also eliminate various contaminants in order to produce saleable products.
Refineries generally prefer sweet crude oil due to its low sulfur content and relatively high yields it produces of high-value products such as gasoline, diesel fuel, heating oil and jet fuel.
Current world environmental regulations strictly limit the sulfur content in refined fuels such as diesel and gasoline.
The majority of the sulfur in crude oil occurs bonded to carbon atoms, with a small amount occurring as elemental sulfur in solution and as hydrogen sulfide gas.
Sour oil can be toxic and corrosive, especially when the oil contains higher levels of hydrogen sulfide, which is a breathing hazard.
For safety reasons, sour crude oil needs to be stabilized by having hydrogen sulfide gas (H2S) removed from it before being transported by oil tankers.
Since sour crude is more common than sweet crude in the U.S. part of the Gulf of Mexico, Platts has come out in March 2009 with a new sour crude benchmark (oil marker) called Americas Crude Marker (ACM).
Dubai Crude and Oman Crude, both sour crude oils, have been used as a benchmark (crude oil) oil marker for Middle East crude oils for some time.
The major producers of sour crude oil include:
North America: Alberta (Canada), United States' portion of the Gulf of Mexico, Alaska and Mexico.
South America: Venezuela, Colombia, and Ecuador.
Middle East: Saudi Arabia, Iraq, Kuwait, Iran, Syria, and Egypt.