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Fossil Fuel Formation: Looking to the Past to Plan for the Future

Powering Modern Civilizations

Fossil fuels are the main source of energy that have powered modern human civilizations for nearly three centuries. According to the National Academies of Sciences, fossil fuels like crude oil, coal, and natural gas make up about 81 percent of the total energy used in the United States (Brown, 2019). Similarly, Edward Rubin, a professor of environmental engineering and public policy at Carnegie Mellon University in Pittsburgh, estimates that roughly 80 to 85 percent of the total global energy supply is powered by fossil fuels (Metcalfe, 2019). Fossil fuels have become a critically important component for everyday life and have contributed immensely to the development of the modernized world.

Virtually all of the fossil fuels that are used today, including crude oil, coal, and natural gas, were formed millions of years ago during the prehistoric era when dinosaurs roamed the Earth. However, a common misconception about fossil fuels is the notion that these sources of energy actually started out exclusively as dinosaurs.

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Millions of Years of Formation

Fossil fuels were formed over millions of years as a result of decomposing plants and animals. Even though some dinosaurs could have technically been transformed into fossil fuels over many millennia, the vast majority of fossil fuels were formed from green plants that died in the perfect conditions, which included an oxygen-free (anoxic) environment and an extensive amount of time. According to Azra Tutuncu, a petroleum engineer and geoscientist at the Colorado School of Mines, fossil fuels can be formed from any number of carbon-based organisms within the right environmental conditions (Zielinski, 2018).

The energy from fossil fuels is stored within the bonds between an organism’s molecular atoms. Burning fossil fuels ignites a process that breaks apart an organism’s bonds, which in turn releases energy. As plants soaked up the sun’s energy through the process of photosynthesis, they locked in solar energy from the sun. When the plants died and their remaining debris gradually sunk below the ground, the energy and carbon became locked in within layers of sediment and rock, only to be rediscovered and utilized millions of years later by modern humans. Over millions of years, the remaining debris from plants and other organisms became intensely compacted and moved deeper into the Earth’s crust, where extremely high temperatures helped to break down the organic matter and transform it into fossil fuels.

Fossil fuels are typically found below the Earth’s crust, trapped within layers of bedrock and non-porous sediment. Layers of coal are often located just 200 to 300 feet below the ground, while oil and natural gas deposits are usually located over a mile or two below the surface, with some oil and gas wells reaching depths of well over six miles under the ground (Metcalfe, 2019). Coal beds are typically found within many layers of sedimentary rock deposits that have piled up over millions of years to provide a massive amount of heat and pressure to decaying plant and animal matter. Over 50 percent of a piece of coal’s weight can be directly attributed to fossilized plants (Brown, 2019).

Source: Wikipedia

Oil and Gas

Oil and natural gas are found between layers of sedimentary rock, such as shale. Oil in its raw and unprocessed form is comprised of a thick petroleum-based sludge that must be heated and refined to make gasoline and diesel fuel. Natural gas is usually found above oil deposits within the form of methane, which is the most abundant organic compound found on Earth (Brown, 2019). Similar to oil and coal, natural gas can also be found deep within sedimentary rock deposits. While gas deposits are often extracted from deep below the Earth, natural gas can also be formed by tiny microorganisms called methanogens. In addition to being found within the intestines of live animals and humans, methanogens can also be found in low-oxygen areas that are often just below the surface of the Earth. For example, the decomposing matter found in landfills is a hotbed for methanogen activity that functions to break down waste and emit a type of methane known as biogenic methane.

Fossil fuel deposits can be found all around the world, but some countries are known to have more proven reserves than others. While the United States, China, and Russia are known to have the world’s largest deposits of coal, nearly half of all the world’s crude oil and natural gas reserves are located in the Middle East (Metcalfe, 2019). According to the U.S. Energy Information Administration, petroleum resources are extracted within 32 U.S. states, with Alaska, California, New Mexico, North Dakota, and Texas being the largest producers (Metcalfe, 2019). Just over 40% of U.S. crude oil production comes from Texas alone.

The plants, animals, and other organisms that have been transformed into fossil fuels originally lived during the Carboniferous Period, around 300 to 360 million years ago (Brown, 2019). During this period of time, the global climate was much warmer and wetter. The algae, bacteria, and other tiny organisms that roamed the Earth during the Carboniferous Period contributed to the ideal conditions that enabled for the formation of fossil fuels. These small organisms, along with the debris from larger plants and animals became compressed underground over tens of millions of years, slowly being converted into a vital mineral called kerogen.

Source: Wikipedia

The Importance of Kerogen

The formation of kerogen is a key component of the fossil fuel development process. Following a lengthy process of years of compacting carbonaceous matter (plants, animals, and other organisms that contain carbon), the complex and waxy mixture of hydrocarbon compound known as kerogen began to form. Kerogen is a naturally occurring insoluble organic material that is of prime importance for the formation of oil, coal, and natural gas. This organic compound is thought to have originated from compressed organic matter, like algae and various other forms of ancient plant life that formed in layers at the bottom of prehistoric lakes and seas and was buried deeply over extensive periods of geologic time.

Many geochemists hypothesize that kerogen may be the source material from which petroleum and other fossil fuels were formed (Curley, 2019). Under the right conditions, as kerogen is buried deep below the ocean floor and subjected to further heat and pressure, it can become transformed into petroleum. Furthermore, if the kerogen had been put under even more stress, pressure, and heat, it would be transformed into natural gas (Zielinski, 2019). On the other hand, some scientists believe that kerogen should just be considered as another type of fossil fuel that was formed during the same period of time and under similar conditions as petroleum and natural gas.

As hydrocarbons from kerogen were transformed into fossil fuels like petroleum and natural gas, they gradually became less dense than the sediment and water within the Earth’s crust, which prompted them to migrate upward until they became trapped by layers of rock. As an increasing amount of fossil fuels become trapped under a layer of bedrock, they form into a reservoir that can eventually be extracted if discovered by humans. When humans drill down through layers of sediment and rock, this allows trapped oil and gas deposits to continue to rise upward, making them easier to extract.

Source: Wikipedia

Coal Formation

While oil and natural gas are thought to have formed mainly from kerogen, the process involved in the formation of coal is different. Most coal deposits were formed between 540 million to 65 million years ago (Metcalfe, 2019). The creation of coal originated in ancient peat bogs and swamps, where lush greenery sunk into wet areas, became partially decayed, and eventually turned into peat (partially decayed vegetation and organic matter) just below the Earth’s surface. After the ancient wetlands and swampy marshes gradually became converted into drylands, a slew of other organic materials started to accumulate over the peat, forcing it deeper underground. Similar to the transformation of kerogen into oil and gas, heat and intensifying pressure over a geologic timeframe contributed to the conversion of peat into coal (Zielinski, 2018).

Coal is often thought of as the most “fossil-y” of the fossil fuels because ancient vegetation is frequently found to be perfectly preserved within layers of coal. Since coal deposits are essentially petrified peat swamps formed from decaying plant life, it is common to find imprints of fossilized plants and even entire petrified trees within coal deposits. The Riola and Vermilion Grove coal mines in eastern Illinois are thought to be two of the world’s most well-known coal mines that hold a snapshot of life on Earth from roughly 300 million years ago.

Source: Wikipedia

The Riola and Vermilion Grove Coal Mines

Following a thorough examination by the Illinois State Geological Survey, the Riola and Vermilion Grove coal mines were found to hold about four square miles of tropical wilderness preserved 307 million years ago, making it the largest intact fossil forest ever found (Gugliotta, 2009). Scientists hypothesize that an ancient earthquake abruptly covered the former tropical swamp with 15 to 30 feet of mud and sand, which essentially locked the forest in place for millions of years. The mines are located about 230 feet below the ground and are filled with intertwined and petrified tree trunks, fern fronds, leaves, and twigs that became frozen in time following a catastrophic earthquake. Following the opening of the mine in 1998, routine inspections unearthed the findings. Paleobotanists (scientists that study the branch of botany that deals with the identification and recovery of plant remains from geological contexts) from the Smithsonian’s National Museum of Natural History described the site as “a spectacular discovery” because the whole ancient forest still remains intact within the ceiling and walls of the coal mines (Gugliotta, 2009).

The Riola and Vermilion Grove coal mines have locked in unique plant species discoveries, like the lycopsids, which were scaly plants that grew upwards of 120 feet in height and had trunks in excess of six feet in circumference. Because coal deposits formed primarily from plants and other organic materials, very few fossilized animals have been found in the Riola and Vermilion Grove mines. Moreover, chemicals found in ancient peat bogs were thought to have dissolved any remaining animal bones or shells. Although, some giant dragonflies with wingspans of two and a half feet, millipedes, spiders, cockroaches, and amphibians have been found.

Source: Wikipedia

Studying the Past to Inform the Future

The discovery of these ancient sites provides a unique opportunity for researchers to study the composition of ancient forests that thrived in a time when atmospheric carbon dioxide was much higher than it is today. Evaluating the relationship between ancient atmospheric changes in carbon dioxide and subsequent changes in vegetation could provide insight about how today’s ecosystems will adapt to increasing levels of atmospheric carbon dioxide that have been caused by the burning of fossil fuels. Additionally, scientists have also been able to analyze the chemical composition of the coal deposits in the Riola and Vermilion Grove mines to develop measures of ancient carbon dioxide, rainfall, temperature, and numerous other variables.

Studying the composition and formation of fossil fuels has provided scientists with vital insights into the Earth’s natural resources, including oxygen levels, water resources, minerals, plants, soil, and animal life. Research related to fossil fuel deposits could continue to help humanity with the identification of opportunities to better adapt to a changing climate. Since fossil fuels are projected to remain as the primary source of the world’s energy for decades into the future, carbon emissions from the consumption of fossil fuels is also expected to continue to increase. In a 2018 report from the United Nations, climate change researchers issued a dire warned about how the world will face a potentially catastrophic level of global warming if carbon emissions from fossil fuels aren’t drastically reduced (Metcalfe, 2019). While this grim projection from the United Nations has sent disconcerting shockwaves through the global environmental community, continued research into ancient atmospheric conditions could reveal opportunities to mitigate some of the impacts of climate change.


Brown, T. (2019). “Fossil Fuels” National Geographic.

Curley, R. (2019). “Kerogen.” Encyclopedia Britannica.

DOE. (2019). “Fossil.” U.S. Department of Energy.

Gugliotta, G. (2009). “The World’s Largest Fossil Wilderness.” Smithsonian.

Metcalfe, T. (2019). “What are fossil fuels?’ NBC News.

Zielinski, S. (2018). “Explainer: Where fossil fuels come from.” Science News.

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