If you follow the oil news, you often hear terms such as sweet and sour oil.
You also probably noticed that sweet oil is usually more in demand and is priced higher than sour crude.
So what makes oil sour or sweet?
Sour crude contains hydrogen sulfide (H2S) while sweet oil doesn’t.
Usually, oil with H2S content above 0.5% is considered to be sour.
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A long time ago people used to taste oil to check if it contained H2S.
The crude without H2S has a slightly sweet taste and that’s where the name came from.
This technique of tasting oil is not used anymore and there are safer and more accurate ways to determine whether oil contains hydrogen sulfide or not.
H2S is a colorless, extremely toxic, and flammable gas that is soluble in oil.
When it is dissolved in water, it forms a weak acid that can corrode metals.
At low concentrations, hydrogen sulfide has the smell of rotten eggs.
Sour oil is priced lower than sweet crude because it is more challenging and expensive to refine.
Sulfur needs to be removed from the oil before it can be turned into fuels and other light distillates.
In addition, processing and transporting sour crude oil is more complicated because of the hazards associated with hydrogen sulfide.
Depending on the regulations sometimes sour crude oil is stabilized before it is transported to refineries or storage facilities.
This involves distillation to remove H2S and sweeten the oil.
There are some oil fields that only produce sweet oil and others that produce sour crude.
It is also possible for the field to start as sweet and slowly turn sour.
This sometimes happens when sulfur-reducing bacteria are introduced into the reservoir.
Examples of sweet oil benchmarks are WTI and Brent.
Western Canadian Select (WCS) and Dubai Crude on the other hand are slightly sour.
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