Oil and natural gas: What are they and what makes them different?

At first blush, the difference between oil and natural gas seems relatively straightforward. One of them is a gas and the other a liquid, making them fairly easy to distinguish.

But that simple distinction offers little understanding of the different sources and applications of the two fossil fuels. Indeed, a little investigation into gas and oil shows how many similarities these two hydrocarbons actually share.

What they are

The place where the differences between oil and gas are most obvious is at the molecular level.

Just out of the well, natural gas contains a variety of different hydrocarbons – such as relatively complex gases like ethane, propane, butane and pentane – along with contaminants like carbon dioxide and water. But the greatest part of natural gas – around 80 percent – is the simple hydrocarbon known as methane, consisting of a single carbon atom attached to four hydrogen atoms.

Oil, or more specifically petroleum, comprises a diverse collection of much more complex hydrocarbons. Several different types of molecules – from alkanes and cycloalkanes to aromatic hydrocarbons and asphaltenes – combine long chains, rings or planes of carbon atoms with attached hydrogen atoms. With every deposit of crude oil having its own mix of chemicals, the fossil fuel requires extensive refining for many uses in modern technology.

Where they come from

Like America’s longtime fuel of choice, coal, both oil and natural gas are referred to as fossil fuels. This term can be taken quite literally.

Some ancient plants and animals survived for modern scientists to dig up in the form of preserved bones, imprints in rock or even entire petrified plants. Others, however, found themselves buried under many tons of sand and rock, creating intense heat and pressure.

In many cases, this pressure eventually caused more solid decaying plant matter to harden into coal. But for smaller organisms like zooplankton and algae, this pressure eventually caused the complex organic matter to break down into more basic structures. This breakdown led to the creation of two different substances, the heavier and more complex carbon formations that compose crude oil and the simpler structures of natural gas.

Getting to the surface

In either case, the substances would eventually well up from the deep pockets in the earth to move through permeable stone, types of rock that allow fluids to pass through them. While oil is far heavier than natural gas, both are less dense than water, allowing them to push up through layers of rock, being replaced by groundwater, until they reach either the surface or a layer of impermeable stone.

With the rise of shale gas in the U.S., many people will be somewhat familiar with this idea, but this same process actually plays a role in the formation of the full range of different oil and gas deposits. The U.S. Energy Information Administration provides a convenient summary of the different types of deposits that can form based on the movements of gas and oil under the surface.

Traditionally, oil and natural gas usually came hand-in-hand in the form of “associated” gas that filled caverns above oil deposits. Oftentimes this gas was simply ignored, but recently greater efforts have been made to capture it. Deeper down, some gas remained trapped in small pockets within shale rock, requiring gas companies to “fracture” the rock before it can be recovered. While coal generally condensed into a solid, some of that matter also broke down into simpler components like methane, leading to the creation of coal-seam gas.

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