Fossil Fuel Components of Crude Oil and What Makes Them Different

While the notion of a barrel of oil is just as common in everyday vernacular as the idea of a bottle of water, what crude oil is made of is a mystery to most people. “What is crude oil made of?” is the question that requires a lengthy explanation, even though it is also very simple — and accurate — answer. 

The EPA’s textbook answer to the question what is crude oil is — “crude oil is unrefined petroleum.” The EPA’s definition is akin to saying ice cream is a frozen dairy product.

While true, the answer does not begin to explain what crude oil is made of. 

A more accurate, though equally elementary, the answer is: crude oil is a heterogeneous mixture of hydrocarbons. Still, that does very little to detail the components of crude oil. The reason a more in-depth explanation is necessary is that the valuable components in crude oil — hydrocarbons — fall into different categories and types. 

The catch is, the answer to, “what is crude oil made of?” is very complex. It is complex because there are so many different types of hydrocarbons that the actual number is unknown, and though hydrocarbons are only made of only two elements — hydrogen and carbon, — crude oil also contains varying degrees of nitrogen, sulfur, metals, oxygen, and other contaminants. So, while the simple answer to, “what is crude oil made of,” is hydrocarbons, the answer to the question “what hydrocarbons are in crude oil?” is complex.

Complex though the answer may be, a general idea of the categories and types of hydrocarbons is important. Knowing what the categories and types of hydrocarbons are in crude oil is important because the categories and types of hydrocarbons are the reason there are different liquid fossil fuels. Because of the different categories and types of hydrocarbons in crude oil, we have available to us: diesel, gasoline, kerosene (jet fuel), heating oil, bunker fuel, etc.

What Other Value do Hydrocarbons Have?

The different types of hydrocarbons in oil are the reason refining a barrel of oil produces so many different types of liquid fossil fuels and oils. And, hydrocarbon categories and types are the reason there are so many other types of petroleum-based products. The hydrocarbons in a barrel of oil are the reason we have synthetics like plastic, fiberglass, carbon fiber, rubber, wax (crayons), polyester, laminates, cosmetics, etc. 

Almost everything synthetic is a product of fossil fuel hydrocarbons. The hydrocarbons in a barrel of oil constitute the chemical construct of everything from pharmaceuticals, antihistamines, lipstick, dentures, balloons, to charcoal briquettes. 

Importance-of-Hydrocarbons Thought Experiment

One way of examining the massive role hydrocarbons from crude oil play in our lives is to thinks of what would be missing from the game of football if fossil hydrocarbons were to disappear. Without hydrocarbons, there would be no helmets, helmet visors, jerseys, shoulder pads, mouth guards, jockstraps, pants, or shoes. Artificial turf is made from the hydrocarbons found in oil. So is athletic tape. So is eye black. In other words, without hydrocarbons, football would be played in socks.

Hydrocarbons are the single most important utility commodity in the post-industrial world. 

What are Hydrocarbons?

The simple answer to, “what is crude oil?” — the answer being hydrocarbons, — prompts another question, “what are hydrocarbons?” As the name implies, hydrocarbons are molecules made of carbon and hydrogen bonds and chains. 

What are the Primary Hydrocarbons in Crude Oil?

There is a simple answer to, “what is oil?” There is an equally simple answer to, “what are hydrocarbons?” But, the question, “what hydrocarbons are in crude oil,” is less straightforward. While the valuable components of crude oil are hydrocarbons, what hydrocarbons are in crude oil is not even known in its entirety by scientists. As such, any answer to that question will — necessarily — be incomplete. 

Not only are there different types of hydrocarbons, each hydrocarbon type has a different energy density. Because every type of hydrocarbon has a different energy density, each type of hydrocarbon in a barrel of crude oil has a different monetary value. That means different categories and types of hydrocarbons have different prices.

The reason there are so many different types of hydrocarbons in crude oil — theoretically, almost an unlimited number, — because there is almost an unlimited number of different ways carbon and hydrogen can combine to make hydrocarbon molecules and molecular chains.

Kinds of Crude Oil

Because there are different types of hydrocarbons and those hydrocarbons are found in different ratios in the crude oil found in different regions throughout the world, there are also different kinds of oil.

“Because of the differences in composition, correlations developed from regional samples, predominantly of one chemical base, may not provide satisfactory results when applied to crude oils from other regions,” explains PetroWiki.com

There are three main kinds of crude oil: light, medium, and heavy. The three main kinds of crude oil can be differentiated not only by their chemical makeup but their appearance and viscosity. Light crude oil, “flows fluidly and is normally of a light golden hue.” Heavy crude oil a, “dark color, very viscous and sticky to the touch.” 

Crude oil types are typically lumped together with respect to the region from which they are taken. That means there are hundreds of types of crude oil because every barrel of crude oil from every well from every region on Earth is different. Still, to simplify the categorization of crude oil, there are only a few dozen types that have been tracked historically. 

There are only five types of crude oil the oil industry recognizes universally: Boscan, Maya, Arabian Light, West Texas Intermediate, and Bass Straight. All other crude oil types typically fall into the specifications of one of these five types.

Though there are different kinds of crude oil, most crude oil types have relatively similar carbon and hydrogen makeups. “Regardless of variations, however, almost all crude oil ranges from 82 to 87 percent carbon by weight and 12 to 15 percent hydrogen by weight,” according to Encyclopedia Britannica. “Those elements form a large variety of complex molecular structures, some of which cannot be readily identified.” 

It is how hydrogen and carbon combine to make fuel molecules and molecular chains that determine the types of hydrocarbons in crude oil. And, differences in crude oil types lead back to the same question, “what are the different hydrocarbons found in crude oil?” 

Why the Differences in Hydrocarbon Types in Crude Oil?

In its natural state, crude oil is a liquid, a fuel that is the product of three components: biomatter, heat, and pressure. Oilprice.com explains, 

“Most of the oil we extract today comes from the remains of prehistoric algae and zooplankton whose remains settled on the bottom of an Ocean or Lake. Over time this organic material combined with mud and was then heated to high temperatures from the pressure created by heavy layers of sediment. This process, known as diagenesis, changes the chemical composition first into a waxy compound called kerogen and then, with increased heat, into a liquid through a process called catagenesis.”

Different types of biomatter — plant or animal tissue — exposed to varying amounts of heat and pressure, a consequence of being buried under sediment and rock, resulting in an incredible variety of different types of hydrocarbons. 

Percentages of Different Categories of Hydrocarbons in Crude Oil

The amount of carbon and hydrogen in crude oil do not vary greatly from one barrel of oil to the next. The percentages of different hydrocarbon types, on the other hand, vary greatly. There are four categories of hydrocarbon molecule chains present in a barrel of oil: alkanes, naphthenes, aromatics, and aliphatics. It is with these four hydrocarbon molecular chains that crude oils vary the most.

• • Alkanes make up an average of 30% of a barrel of oil, but the range is anywhere from 15% to 60%. 
  • Naphthenes make up about 49% of a barrel of oil, but the range is anywhere from 30% to 60%. 
  • Aromatics constitute an average of 15% of a barrel of oil, but composition can range anywhere from 3% to 30%
  • Asphaltics average about 6% of the makeup of a barrel of oil.

The four primary hydrocarbon types have sub-types and that is the reason there are thousands of different hydrocarbons in a single barrel of oil. But, the four categories are relatively uniform — across the board — in all crude oil.

The significance of Differences in Hydrocarbon Chains and Molecules

The greater the number of carbon atoms in relation to hydrogen atoms, the heavier the hydrocarbon atom. The reason being, the higher the carbon-to-hydrogen ratio, the greater the energy density of a fuel. Low energy density fuels like natural gas (methane) and propane, are typically gas fuels. The fossil fuels found in crude oil have higher carbon-to-hydrogen ratios than gas fuels and, therefore, have higher energy densities and are found in liquid form. 

How Many Different Fossil Fuels are found in Crude Oil?

Since it is impossible to account for every hydrocarbon type found in a barrel of crude oil, an easier way of asking what crude oil is made of is to ask what fossil fuels are found in crude oil. Since all the hydrocarbons found in crude oil are — technically — fossil fuels, the question is loaded because we really don’t know the answer. 

However, by breaking down a barrel of oil into the fossil fuel types we recognize, it is easier to understand what — essentially — a barrel of oil contains.

According to the EPA, on average, a barrel of oil is made up of 22 percent gasoline, 11 percent ultra-low sulfur distillate (diesel), 4 percent jet fuel (kerosene), 1 percent heavy fuel oil (bunker fuel), 1 percent high sulfur distillate (heating oil), and 6 percent of a barrel of oil is made up of other products. 

What Hydrocarbons are Different Fossil Fuels Made of?

Again, hydrocarbon combinations determine the type of liquid fossil fuel, gas, or solid (coal) a petroleum product is. 

Gasoline, for example, is — roughly — 21% cycle-hexane, 17% iso-octane, 16% iso-pentane, 16% ethyl benzene, 15% toluene, 12% n-decane, 3% naphthalene, and all other <1%.

Diesel contains different types of hydrocarbons than gasoline, “If the structural analysis is also considered, a mean molecular formula can be found (i.e., with the whole number of atoms and typical carbon-chain length, as C11H21, or C12H23, or C12H26, or C13H26, or C14H30; dodecane and tridecane are the most usual surrogates).”

Fuel oil, heavy fuel oil (bunker fuel), jet fuel (kerosene), natural gas (methane), propane, etc., are all made of different types of hydrocarbons.

Which Fossil Fuel is the Most Valuable?

There are a number of ways of valuing fossil fuel hydrocarbons. One way is to simply take the market value of a particular fossil fuel type and compare it to the rest. In that case, kerosene is probably the most valuable followed by the conglomerate of other hydrocarbons in a barrel of oil that are used to produce things like plastics and cosmetics. Low sulfur diesel and gasoline are somewhere in the middle of the price range. At the bottom are heating oil, heavy fuel oil, and hydrocarbon gas liquids — liquid natural gas and propane, for example. 

But with respect to energy potential, diesel, heating, and heavy fuel oil, and kerosene are the most valuable. While gasoline has the largest fossil fuel market share, it is a relatively inefficient fossil fuel on a volume scale. Gasoline does not have a high energy potential in relation to many of the other fossil fuels found in a barrel of crude oil. 

Which Fossil Fuels in Crude Oil Have the Highest Fuel Density?

The fossil fuels with the highest density are fuel oils, kerosene, anthracite coal, and diesel. In the mid-range are the different octane grades of gasoline as well as low-grade coals. The least dense fossil fuels are those fossil fuels that are gases like methane and propane. 

Which Fossil Fuel Hydrocarbons in Crude Oil are the Most Polluting?

There is a misconception that those fuels with the most fuel density — meaning the carbon-to-hydrogen ratio is high — pollute the most. And, on a volume scale, they do. But, that does not mean high-energy density fuels are bigger polluters. The reason low-energy fuels like gasoline and gas-state fuels are actually bigger polluters is that more low-energy fuel must combust before a requisite amount of energy is produced.

Take diesel versus gas as an example. Though diesel emits slightly higher emissions on a volume scale, a diesel powered vehicle travels between 25% and 35% farther than a gas engine. In other words, if a gallon of diesel produces 5% more emissions than a gallon of diesel, but a gasoline engine requires 30% more fuel to travel an equal distance, that means over the course of a trip, the gasoline engine will produce far more emissions than the diesel engine. 

In short, the value of crude oil is found in hydrocarbons. But, not all hydrocarbons are created equal. What is crude oil made of? Crude oil is a mixture of high and low-value hydrocarbons.