The Geological Journey of Fossil Fuels: How Coal, Oil, and Natural Gas Formed

Every time coal is burned in a power plant or oil is refined into gasoline, the energy released was originally captured from sunlight hundreds of millions of years ago. Fossil fuels – coal, petroleum, and natural gas – are not simply buried rocks or liquids. They are the chemically transformed remains of ancient life, locked away through slow geological processes that took place over timescales almost impossible to comprehend. Understanding how they formed helps us appreciate both the remarkable science behind them and the environmental consequences of extracting and burning them at the pace we do today.

Coal formation and its geological history 🔗

Coal begins as plant matter. Specifically, it starts as peat – partially decomposed vegetation that accumulates in waterlogged, swampy environments called mires. This process requires a setting where organic matter builds up faster than it can fully decompose, typically in warm, humid conditions with standing water that limits oxygen exposure and slows bacterial breakdown.

The most productive period for coal formation was the Carboniferous Period, which spanned roughly 359 to 299 million years ago. During this time, vast swampy forests covered much of what is now North America and Europe. Enormous tree-like plants – including giant lycopsids and early ferns – grew in dense equatorial wetlands. When they died, their organic matter accumulated in thick layers rather than decomposing entirely, partly because wood-digesting microbes had not yet fully evolved, and partly because the tectonic conditions of the time created deep subsiding basins that allowed organic material to pile up and become buried.

Stanford researchers have shown that the formation of Pangea played a direct role in coal accumulation – as tectonic plates collided to form the supercontinent, mountain ranges rose and adjacent basins deepened, giving organic debris enough time and space to accumulate before being buried by sediment. This geological framing, rather than biology alone, now appears central to explaining why so much coal formed during this specific window of Earth’s history.

Research published in PNAS estimates that roughly 90 percent of the coal burned for energy today was deposited during the Carboniferous period – a fact that underscores both the geological concentration of these deposits and their climatic significance, since their formation drew enormous quantities of carbon dioxide out of the atmosphere.

From peat to anthracite: the coalification stages 🔗

Once plant matter is buried under layers of sediment, it undergoes a process called coalification – a gradual transformation driven by increasing heat and pressure with depth. This progresses through distinct stages, each representing a higher grade of coal with greater carbon concentration and lower moisture content:

• Peat – the raw starting material; less than 60% carbon, high in moisture and volatile organic compounds. Technically not yet coal.
• Lignite (brown coal) – 65-70% carbon; low-grade, high moisture. Found in younger deposits that have not been deeply buried.
• Sub-bituminous coal – 70-76% carbon; used mainly in industrial boilers.
• Bituminous coal – 70-86% carbon; the most widely used grade, including for steel production via coking.
• Anthracite – the highest rank, with over 90% carbon. Hard, shiny, and slow-burning. Formed under the most intense heat and pressure.

The rank a coal deposit reaches depends on how deeply it was buried, for how long, and the temperatures it was exposed to. According to Britannica, the three key physical factors controlling coalification are duration, increasing temperature, and increasing pressure – with microbial activity ceasing within a few metres of the surface, after which purely physical forces take over the transformation.

Petroleum: from ancient marine organisms to crude oil 🔗

Oil and natural gas have a fundamentally different origin from coal. Rather than land plants, they derive primarily from microscopic marine organisms – mainly phytoplankton and zooplankton – that lived in shallow seas millions of years ago. The common assumption that oil comes from dinosaurs is a myth. These microorganisms predate the dinosaurs by hundreds of millions of years, and it is their organic remains, not large animal carcasses, that gave rise to the world’s petroleum deposits.

Around 70% of existing oil deposits formed during the Mesozoic era (252 to 66 million years ago), a period marked by warm tropical oceans and exceptionally high levels of marine biological productivity. When these microorganisms died, their remains sank to the seafloor and mixed with fine sediment in low-oxygen (anoxic) environments, where decomposition was limited. Over time, layers of silt and mud buried this organic material deeper and deeper.

The kerogen stage 🔗

During the first stages of burial, the organic matter undergoes chemical and physical changes that transform it into a waxy, insoluble solid called kerogen. This typically happens in the first kilometer or so of burial, as temperature and pressure begin to reorganize the organic molecules into a three-dimensional carbon-and-hydrogen network within the host rock.

Kerogen type matters for what comes next. Kerogen derived from algal and marine organisms (Type I and II) is oil-prone, while kerogen from terrestrial plant material (Type III) tends to produce gas rather than liquid oil.

From kerogen to crude oil and natural gas 🔗

As burial deepens to around 2-4 kilometers, the temperature of the source rock increases due to the Earth’s natural geothermal gradient. When the rock reaches approximately 100-150°C, the kerogen begins to crack – breaking apart into smaller hydrocarbon molecules that make up crude oil. This stage is called catagenesis.

If temperatures rise further, typically above 150-160°C, the heavier oil molecules break down into shorter, lighter hydrocarbons, producing natural gas – primarily methane. The oil window sits at roughly 60-120°C, while the gas window lies between 120-180°C, corresponding to depths of 2-4 km and 4-6 km respectively. This is why natural gas is often found in the same geological formations as oil, but at greater depths or from more thermally mature source rocks.

Once generated, oil and gas migrate upward through porous rock due to buoyancy – oil is less dense than water and naturally floats upward through water-saturated rock layers. When this upward movement is blocked by an impermeable cap rock, the hydrocarbons accumulate in what geologists call a trap or reservoir. It is these underground reservoirs – not liquid-filled underground lakes – that drilling operations access.

Extraction and processing of fossil fuels 🔗

Getting fossil fuels out of the ground requires fundamentally different techniques depending on the fuel type. For solid coal, mining is used. For liquid oil and gaseous natural gas, drilling is the primary method.

Coal mining methods 🔗

Two main methods are used to extract coal: underground (deep-shaft) mining for deep deposits, and surface (opencast or strip) mining for shallower ones. Surface mining has become increasingly common in recent decades. Its most invasive forms include area strip mining, where overlying soil and rock are removed in wide strips to expose coal seams, and mountaintop removal, a technique that literally blasts away the tops of mountains to access coal beneath.

Underground mining poses acute risks to worker safety – cave-ins, methane explosions, and long-term respiratory diseases such as black lung (coal workers’ pneumoconiosis) are well-documented hazards. Surface mining, while mechanically safer, causes extensive land degradation, destroys vegetation and habitats, and generates toxic runoff that can contaminate local water sources with selenium, arsenic, manganese, and other heavy metals.

Oil and gas drilling 🔗

Oil and gas are typically accessed by drilling wells into underground reservoirs. Conventional drilling involves vertical wells into known reservoirs. More recently, horizontal drilling allows a single well to extend laterally through a reservoir, reaching a much larger volume of rock from one surface location. Hydraulic fracturing (fracking) takes this further – high-pressure fluid is injected into rock to fracture it and release trapped oil or gas. Fracking has been linked to groundwater contamination and localized earthquakes, and generates large volumes of chemically laden wastewater that must be managed carefully.

Offshore drilling introduces additional risks. Between 2008 and 2012, offshore rigs in the U.S. alone experienced 34 fatalities, over 1,400 injuries, and 60 significant oil spills. High-profile disasters like the 2010 Deepwater Horizon blowout demonstrated the scale of ecological damage that can result when deep-water extraction goes wrong, causing extensive harm to marine and coastal ecosystems across the Gulf of Mexico.

Processing and refining 🔗

Raw fossil fuels require significant processing before use. Crude oil is sent to refineries where it is separated into products like gasoline, diesel, kerosene, jet fuel, and petrochemicals through a process called fractional distillation. Coal is cleaned and sorted by grade before being burned in power plants or converted to coke for steel production. Natural gas is typically processed at the wellhead to remove water, impurities, and heavier hydrocarbons before being piped to consumers.

Environmental considerations 🔗

The environmental costs of fossil fuel extraction are significant and span the entire production chain. Coal mining results in land damage and releases pollutants including sulfur dioxide and mercury into surrounding ecosystems. Oil extraction carries the constant risk of spills that contaminate both freshwater and marine environments. Fracking fluids have been found to leach into drinking water aquifers. And beyond the extraction site itself, the roads, pipelines, and infrastructure associated with fossil fuel development fragment habitats and push wildlife into smaller, more isolated areas.

The International Energy Agency (IEA) estimates that as of 2023, fossil fuels still account for around 80 percent of primary energy production worldwide. Burning them releases carbon dioxide far faster than natural processes can absorb it – a direct driver of the climate change now reshaping the planet’s ecosystems. The irony is that the very process which drew carbon dioxide out of the atmosphere hundreds of millions of years ago – the mass burial of organic matter that formed coal and oil – is being rapidly reversed by the industrial combustion of those same deposits.

What do you think? Given that fossil fuels took hundreds of millions of years to form but are being consumed within a few centuries, how should societies balance the economic reliance on these resources against the pace of the transition to alternatives? And does understanding the geological origins of fossil fuels change the way you think about their value or their environmental cost?

References

1. https://energyeducation.ca/encyclopedia/Coal_formation
2. https://www.britannica.com/science/coal-fossil-fuel/Origin-of-coal
3. https://www.nationalgeographic.com/science/article/the-fantastically-strange-origin-of-most-coal-on-earth
4. https://news.stanford.edu/stories/2016/01/coal-formation-pangea-012216
5. https://pnas.org/doi/10.1073/pnas.1712062114
6. https://www.planete-energies.com/en/media/article/how-coal-formed-process-spanning-eras
7. https://uwaterloo.ca/wat-on-earth/news/formation-oil-and-other-elements-required-produce-petroleum
8. https://energyeducation.ca/encyclopedia/Oil_formation
9. https://oilfieldbeginner.com/the-hidden-origins-of-oil-from-plankton-to-petroleum/
10. https://www.offshoreengineering.com/petroleum-geology/hydrocarbon-formation/
11. https://www.ucs.org/resources/how-bad-are-fossil-fuels-actually
12. https://www.nrdc.org/stories/fossil-fuels-dirty-facts
13. https://www.ebsco.com/research-starters/science/fossil-fuels-and-environmental-degradation
14. https://theconversation.com/leave-it-in-the-ground-how-fossil-fuel-extraction-affects-biodiversity-19484