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Description

Crude Oil is a black liquid found in geological formations. It is a fossil fuel, which means it is formed from dead organisms that are buried under intense heat and pressure.

There are three primary qualities that differentiate one oil from another: Weight, Sweetness, and TAN count.

1. Weight of Crude Oil: Light Oil vs. Heavy Oil
   Heavy oil evaporates slowly and contains material that will be used to make heavy products like asphalt.
   Light oil requires less processing and produces a greater percentage of gasoline and diesel than heavy oil.
   The standard unit of measurement for oil weight is API Gravity.
   This scale was created by the American Petroleum Institute to measure the density of oil.
    
2. Sweetness of Crude Oil

   What makes a 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.
   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.
   HIGH Sulfur Level such as  to 2% correspond to Sour Oil, whereas LOW Sulfur Level below 1% correspond to Sweet Oil.

3. TAN Count of Crude Oil
   TAN stands for “Total Acid Number.” The TAN count of oil is a measure of how corrosive it is.
   If a crude has a high TAN number such as 3.0 mg KOH/g, producers must use more robust metallurgy than standard so their processes can handle that corrosivity and keep the crude in the pipe, compared with low TAN count such as 0.01 mg KOH/g that has very corrosivity.

There are over one hundred different crude oils traded on the market today. These oils are typically labeled by the region they come from, and they have a specific chemical makeup.^[1]

Crude Oil Assay

Crude Oil Assay consists of a compilation of data on properties and composition of Crude Oils. The Assay provides critical information on the suitability of Crude Oil for a particular Refinery and estimating the desired Product yields and quality. It also indicates how extensively a given Crude Oil should be treated in a Refinery to produce Fuels that are in compliance with environmental regulations. A typical Crude Assay should include the following major specifications:^[2]

• API Gravity
• Total Sulfur (% wt)
• Pour Point (°C)
• Viscosity @ 20°C (cSt)
• Viscosity @ 40°C (cSt)
• Nickel (ppm)
• Vanadium (ppm)
• Total Nitrogen (ppm)
• Total Acid Number (mgKOH/g)
• Distillation Data
• Characterization factor K_UOP, K_W

A randomly selected, typial Crude Assay is shown in Table 1. 

Table 1: Bakken Crude Summary Report (Crude Assay)^[3]

References

1. Kimray Inc.^®, Types of Crude Oil: Heavy vs Light, Sweet vs Sour, and TAN count
2. PennState College of Earth and Mineral Sciences, FSC 432: Petroleum Processing, Crude Assay
3. equinor, Crude oil assays

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