What Are Fossil Fuels Made Of?

Perhaps the most commonly known fact about fossil fuels is that the emissions they generate when they are burnt represent the largest source of emissions of carbon dioxide, a pervasive greenhouse gas which contributes to global warming and climate change [1]. But the story of their origin involves complex processes in the fascinating setting.

Commonly found underground, fossil fuels were formed by what can only be called a particularly lengthy process over millions of years.

The origin of fossil fuels and the theory that they were formed from the fossilized remains of dead plants over millions of years was first introduced by German scholar Georgius Agricola, also known as “the father of mineralogy”, in 1556 and later by Russian scientist Mikhail Lomonosov in the 18th century. Since the 16th century, we have learnt a lot more about the origin of fossil fuels.

The era of carbon

The age when fossil fuels were formed is called the Carboniferous Period, part of the Paleozoic Era, a period which lasted from 359.2 to 299 million years ago.

“Carboniferous” gets its name from carbon, the basic element in coal and other fossil fuels. To understand how fossil fuels were formed, we need to imagine how our planet looked all these millions of years ago.

300 million years or so ago, the Earth looked very different to what we know today. The land masses we recognise on the world map today were still being formed so swamps and bogs dominated the surface alongside lakes and oceans. Prehistoric plants and trees dominated and the climate was a lot warmer and humid; seasons if any were indistinct.

Scientists came to this conclusion after comparing between fossil and modern-day plant morphology. Based on the fossils that have been found, the Carboniferous plants resemble those that live in tropical and mildly temperate areas today. Tiny one-celled organisms called protoplankton floated in the ocean while creatures unlike those we see today occupied the land.

How were fossil fuels made?

When these ancient living things died, they decomposed and were buried underneath layers and layers of mud, rock, and sand. Eventually, hundreds and sometimes thousands of feet of earth covered them. In some areas, the decomposing materials were covered by ancient seas, then the seas dried up and receded. Over the millions of years that passed, the dead plants and animals slowly decomposed into organic materials and formed what we find today as fossil fuels.

Different types of fossil fuels were formed depending on what combination of animal and plant debris was, how long the material was buried, and what conditions of temperature and pressure existed when they were decomposing [2].

Conservation of ancient energy

While there are several variables that determined the kind of fossil fuel that was created, it was the same natural process that converted dead plants and organisms into coal, oil or natural gas. The process is called anaerobic decomposition which involves microorganisms breaking down biodegradable material in the absence of oxygen; in the case of fossil fuels, it was buried dead organisms, containing energy originating in ancient photosynthesis.

This is important because just like plants today, prehistoric vegetation also sequestered carbon. Therefore, when we burn fossil fuels, we are also upsetting the Earth’s carbon balance, by introducing more CO2 into the atmosphere.

The formation of different types of fossil fuels

Petroleum and natural gas were formed from the remains of organisms including phytoplankton and zooplankton that settled to the bottom of the sear or lakes in large quantities. Over time, this organic matter, mixed with mud, got buried under layers of sediment.

This resulted in high levels of heat and pressure which caused the organic matter to chemically alter, first into a waxy material known as kerogen which is found in oil shales, and then with more heat into liquid and gaseous hydrocarbons in a process known as catagenesis [2,3].

Over time, some of this oil and natural gas began working its way upward through the earth’s crust until they ran into rock formations called “caprocks” that are dense enough to prevent them from seeping to the surface. It is from under these caprocks that most oil and natural gas is produced today.

In contrast, terrestrial plants have mainly helped form coal and natural gas. In some areas, such as what-is-now the eastern United States, coal was formed from swamps covered by sea water. The sea water contained a large amount of sulphur, and as the seas dried up, the sulphur was left behind in the coal.

Some other coal deposits were formed from freshwater swamps which had very little sulphur in them. These coal deposits, located largely in the western part of the United States, have much less sulphur in them [4].

Knowing where fossil fuels come from and how they were made certainly provides great insights into how our planet has been transformed over hundreds of thousands of years as well as the natural processes that came to play over that period.