The first 10 episodes of the Oil 101 podcast have covered the ‘Microbes to Markets’ journey of getting hydrocarbons out of the ground, converting them to useful products, and bringing them to consumers.

Now we turn to the ‘What is…?’ portion of Oil 101, and we start with the most basic of oil and gas questions:

What is Crude Oil?

In this episode of the Oil 101 podcast series, we will discuss what exactly crude oil is and some of the key components of crude oil that affect it being refined into useful petroleum products.

In this 5-minute podcast, we will discuss:

Listen to ‘What is Crude Oil’ below:

It is important to understand a few basic properties of crude oil, the key raw material processed in a refinery.

Crude oils are not all the same, but rather vary in color, composition, and consistency.  They are generally classified as light or heavy , and sweet or sour .  

Unlike water, crude oil is not a chemical compound.  Rather, it is a complex mixture of molecules, consisting of compounds formed from hydrogen and carbon atoms, called hydrocarbons .  Just as water boils at 212 degrees Fahrenheit, each of the compounds in crude oil has its own boiling temperature.  In general, the more carbon atoms in the hydrocarbon molecule, the higher the temperature at which it boils and vaporizes.  It is this phenomenon that permits crude oil to be refined into so many different products.

Scientists usually measure the density of a liquid using specific gravity, which is the weight of the liquid divided by the weight of same volume of water.  However, the popular approach in the oil industry, defined by the American Petroleum Institute (API), for measuring crude oil density is API Gravity , which is expressed in degrees API.

As opposed to specific gravity, API gravity decreases as the density of the compound being measured increases, meaning the higher the API gravity the lighter the compound.  For example, the API gravity of naphtha is about 50 degrees API, but the API gravity of asphalt is around 11 degrees API.

Another important variable in determining the quality of crude is the sulfur content.  Crudes with low sulfur content, less than 0.5%, are referred to as sweet crudes, and those with sulfur contents greater than 2.5% are known as sour crudes.  Crudes with sulfur content in between these values are considered intermediate .  

In the US crude oil volumes are measured in barrels, with each barrel equalling 42 US gallons.  This converts to 35 Imperial gallons in Europe.  This custom stems from the early days of the Pennsylvania oilfields when oil was shipped in 50 gallon wine barrels.  To allow for spillage during transportation, payment at the destination was only for 42 gallons.  Shippers soon learned to seal the barrels and include only 42 gallons.

The origin of the sweet and sour terminology to describe the sulfur content in crude oil has more to do with taste than you may think. In the early days of Pennsylvania crude oil production, petroleum was being used as a substitute for whale oil for indoor lighting.  If a batch of kerosene had too much sulfur, it would emit an unpleasant odor when burned.  The method used to determine if kerosene was suitable for the New York and Philadelphia markets was to taste it.  If the taster thought it sweet, it passed; if it tasted sour, it was rejected as having too much sulfur.

Different oil producing areas yield dramatically different varieties of crude oil. This is why it's common for crude oils to be identified by their origin and characteristics, such as West Texas Intermediate or   Nigerian Bonny Light .

The economic value of a crude increases as its API gravity increases (as it gets lighter ) and as its sulfur content decreases.

Thus a sweet light crude will command a higher price than a sour heavy crude.  This is because light crudes contain more valuable components such as gasoline than do heavy crudes. It is also costly to remove the high content of sulfur in a sour crude. As a result, a refiner will pay more per barrel for West Texas Intermediate than for Venezuelan Pilon.

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