Different crude oil grades are valued differently by refiners based on their crude qualities. There are a large number of different qualities that affect how a refiner will value an individual crude grade.
Crude qualities fall into two categories: whole crude qualities and crude fraction or cut-specific properties.
Whole crude properties:
- API gravity - Also known as density, which is an indicator of the composition of the crude and gives a rough idea of the ratio of different distillation cuts that will result from processing the crude. Generally a lighter, higher API crude is more desirable
- Sulfur content - How sweet or sour the crude is. Sulfur is an undesirable contaminant, so lower sulfur is more desirable. Typically measured in wt %
- Acid number - Also known as TAN, a measure of how corrosive the crude will be when processed. Lower acid is less corrosive and more desirable. Typically measured in Mg KOH/gram of oil required for neutralization
- Nitrogen - Nitrogen is a contaminant that can be harmful to the catalysts used in some conversion processes. Typically measured in wt ppm
- Viscosity - A liquid's resistance to flow at a specific temperature. Higher viscosity ("thicker") crudes are more difficult to pump and less desirable. Typically measured in centistokes (cst) at 20C or 77F
- Pour point - Similar to viscosity, specifically the lowest temperature at which a liquid shows signs of fluid movement. Measured in degrees (C or F)
- Mercaptan - A specific form of sulfur that produces an undesirable smell at even very low concentration. Measured in ppm wt
- H2S - A sulfur gas that is highly toxic in even small concentrations. Measured in ppm wt.
- Metals - Trace amounts of metals that can harm sensitive catalysts. Typically include nickel, vanadium, iron, and mercury. Measured in ppm weight
- Organic chlorides - Measured in ppm
Fraction-specific qualities apply only to individual crude fractions or cuts. Which properties are measured depend on the specific fraction or cut and how it is used in either further processing or direct blending into a finished product.