Oil Definition

So, What Is Oil Anyway?

As silly as it may sound, when asked for an oil definition, often times you get the reply,

“...Oh, you know, the stuff you put in your car.”

Well, that’s somewhat true, but that response actually addresses what it’s used for once it's processed, not what it is.

Let’s see: snake oil, rock oil, black gold, and - Texas tea? Over the years, oil has gone by a number of colorful and descriptive names by both technical and non-technical people.

Technically speaking, oil can be both refined and unrefined. Refined oil is transformed into familiar products such as gasoline, kerosene, diesel fuel, motor oil, etc. (go to refining discussion) . Unrefined oil is known simply as crude oil due to the presence of various amounts of impurities that have mixed with the oil deep down in the earth.

Crude oil typically ranges from black to brown to green in color and can have a waxy feel to it. It also has a strong scent. When oil is produced from wells it is known as crude oil until that time when it is delivered to a refinery and is processed into some of the refined products mentioned above.

But, there are other terms for oil. The most common term not mentioned already is petroleum (the literal oil definition: “petra” = rock, “oleum” = oil). Petroleum is a name pretty much synonymous with oil but differs primarily in that petroleum can be a gas (i.e., vapor), a liquid, a solid, and a semi-solid. These different forms are known as phases.

Therefore, crude oil is a liquid phase of petroleum.

Some of the other terms for petroleum are called natural gas, and bitumen (i.e., asphalt, and tar).

So, what makes the various forms or phases of petroleum related? What do they have in common?

The answer is their chemistry. They are all forms of simple and complex hydrogen and carbon compounds known as hydrocarbons.

Simple hydrocarbon compounds like methane have one carbon atom and four hydrogen atoms (CH4) and are so light that they are a gas in normal atmospheric conditions.

Other hydrocarbon compounds are liquid under normal atmospheric conditions, meaning they are heavier than natural gas but are often lighter than water. This remarkable fact means that oil will float on water, and it is one of the main principles utilized by oil companies to locate and extract crude oil from the earth.

Heavier hydrocarbon compounds still, are solid or almost solid to the point that they will not flow under normal atmospheric conditions thus requiring special techniques to recover them (i.e., bitumen, asphalt, and tar). These types of hydrocarbon can have in excess of 25 carbon atoms.

Hydrocarbon Series

Hydrocarbons have generally been divided into four main series based on differing chemical properties. They are paraffins, isoparaffins, naphthenes, and aromatics.

Paraffins (or alkanes) are straight-chained hydrocarbons, and are typically the predominant hydrocarbon present in gas and liquid petroleum. They represent some of the most common and familiar hydrocarbon compounds beginning from light to heavy: methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), pentane (C5H12), hexane (C6H14), heptane (C7H16), and octane (C8H18). Methane (the most common constituent of natural gas) through butane are gases under normal atmospheric conditions, while pentanes through octane are liquids.

Isoparaffins are molecules that have the same molecular formula composition as their paraffin counterparts, but differ with their branched-chain structures. For example, isobutane (C4H10) has the same number of molecules as butane (C4H10), but because of its different structure, has different physical characteristics.

Naphthenes (cycloparaffins) are closed-ring hydrocarbons that constitute up to 30 percent of straight run (uncracked) gasoline. Cyclopentane (C5H10) and cyclohexane (C6H12) are the most common naphthenes found in petroleum.

Aromatics (benzene) are also closed-ring, but strongly scented hydrocarbons that generally comprise up to approximately 10 percent of crude oils. Benzene (C6H6) is its most common member.


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