Most hydrocarbons on Earth formed from ancient organic matter, mainly microscopic marine organisms, that was buried and slowly cooked by heat and pressure over millions of years. But hydrocarbons also exist far beyond our planet, and humans have learned to make them synthetically. The full story spans deep geology, outer space, and industrial chemistry.
Ancient Marine Life and the Birth of Oil
The hydrocarbons we pump out of the ground as crude oil started as tiny aquatic organisms, mostly plankton and algae, that lived in ancient seas hundreds of millions of years ago. When these organisms died, they sank to the ocean floor and mixed with sediment. Their proteins, fats, and carbohydrates began breaking down through chemical reactions, compaction, and microbial activity within the first several hundred meters of burial. Water was squeezed out, and the organic material transformed into a waxy substance called kerogen and a tar-like material called bitumen.
This early stage, called diagenesis, happens under relatively mild temperatures and pressures. The real transformation begins when the kerogen gets buried deeper. As temperatures and pressures climb, the kerogen starts breaking apart into chains of hydrogen and carbon atoms: hydrocarbons. This thermal cracking process requires a specific temperature window. Too cold, and the organic material stays locked as kerogen. Too hot, and the result is mostly natural gas (very short hydrocarbon chains like methane) rather than the longer chains that make up liquid oil.
Even with the right temperature, forming an oil reservoir that humans can tap requires three things lining up at once. First, there has to be a source rock rich in organic material buried at the right depth. Second, a porous reservoir rock with connected spaces must exist nearby for the oil to flow into and accumulate. Third, a cap rock or seal has to sit on top to prevent the hydrocarbons from migrating all the way to the surface and dissipating. This combination is geologically uncommon, which is why oil deposits are concentrated in specific regions rather than spread evenly across the planet.
Two Ways Natural Gas Forms
Natural gas, primarily methane, comes from two distinct processes. Thermogenic methane forms the same way oil does: through heat breaking down buried organic matter. This typically happens at temperatures above 150°C, deeper in the earth than where oil forms. The higher the temperature, the more the longer hydrocarbon chains crack apart into the simplest one: methane.
Biogenic methane takes a completely different path. Instead of heat doing the work, ancient single-celled organisms called methanogens produce methane as a metabolic byproduct. These microbes thrive in oxygen-free environments like swamps, landfills, and shallow sedimentary layers, generally at temperatures below 80°C. The methane they produce can accumulate in the same kinds of geological traps that hold thermogenic gas, making it commercially extractable. Scientists distinguish between the two types by analyzing subtle differences in the isotopic signatures of the gas, essentially a chemical fingerprint that reveals whether heat or biology did the work.
How Coal Forms From Ancient Forests
Coal tells a different origin story than oil and gas. Instead of marine organisms, coal comes from land plants, particularly the massive ferns, mosses, and trees that grew in swampy forests during the Carboniferous period, roughly 300 to 360 million years ago. When these plants died and fell into waterlogged, oxygen-poor swamps, they didn’t fully decompose. Instead, they accumulated as thick layers of peat.
As sediment piled on top over millions of years, the peat was compressed and heated. This process, called coalification, drives out water and concentrates carbon in stages. Peat becomes lignite (brown coal), the softest and lowest-energy form. With more heat and pressure, lignite becomes sub-bituminous coal, then bituminous coal, and finally anthracite, the hardest and most carbon-rich variety. Each stage represents deeper burial, higher temperatures, and a higher proportion of carbon relative to hydrogen and oxygen. The hydrocarbons in coal are more complex and tightly bound than those in oil or gas, which is why coal is a solid rather than a liquid or gas.
Hydrocarbons Beyond Earth
Hydrocarbons are not unique to our planet. Saturn’s moon Titan has hundreds of times more liquid hydrocarbons than all the known oil and natural gas reserves on Earth combined. On Titan, where surface temperatures sit around minus 179°C, methane and ethane play the role that water plays here. They rain from the sky, fill vast lakes and seas, and collect in dune fields along the equator. NASA’s Cassini spacecraft mapped about 20 percent of Titan’s surface with radar and found several hundred lakes, with dozens of individual lakes each estimated to hold more hydrocarbon liquid than Earth’s entire oil and gas reserves. The equatorial dunes alone contain a volume of organic material several hundred times larger than Earth’s coal reserves.
These hydrocarbons were never alive. They formed through non-biological chemical reactions driven by ultraviolet radiation and other energy sources acting on simple molecules in Titan’s atmosphere. This is a reminder that while Earth’s hydrocarbons are overwhelmingly biological in origin, the chemistry of hydrogen bonding with carbon is fundamental to the universe and doesn’t require life.
The Deep Earth Theory
A minority scientific theory, developed primarily in Russia and Ukraine over the past 50 years, proposes that some hydrocarbons originate not from buried organisms but from deep within Earth’s mantle. The idea is that carbon and hydrogen at extreme depths, roughly 70 to 250 kilometers down, combine under intense pressure and temperature to form hydrocarbon chains that then migrate upward through deep faults into the crust.
Lab experiments have lent some support to the concept. Researchers have produced mixtures of simple hydrocarbons by reacting inorganic ingredients (calcium carbonate, water, and iron oxide) at pressures of 30,000 to 50,000 times atmospheric pressure and temperatures of 900°C to 1,200°C, with no biological material involved. Theoretical calculations suggest these conditions exist naturally at mantle depths.
The theory remains controversial and is not accepted by most petroleum geologists. The overwhelming geochemical evidence, including biological marker molecules found in crude oil, ties the vast majority of Earth’s petroleum to organic origins. Still, the experiments demonstrate that abiogenic hydrocarbon synthesis is at least physically possible under mantle conditions, even if it plays little or no role in the oil and gas deposits we actually extract.
Synthetic Hydrocarbons Made by Humans
Humans can also manufacture hydrocarbons from scratch. The most established method is the Fischer-Tropsch process, developed in the 1920s, which converts carbon monoxide and hydrogen gas into liquid hydrocarbons using iron or cobalt catalysts. The carbon monoxide and hydrogen (together called “syngas”) can come from gasifying coal, biomass, or municipal waste. More recently, researchers have explored capturing carbon dioxide from the air, converting it to carbon monoxide, and then running the same reaction to produce synthetic fuels.
The process works by building up hydrocarbon chains on the surface of the catalyst, essentially assembling fuel molecules atom by atom. Cobalt-based catalysts are favored for producing longer-chain hydrocarbons suitable for jet fuel and diesel. The technology is already used commercially in countries like South Africa and Qatar, though the fuels cost more to produce than simply extracting petroleum. As carbon capture technology matures, synthetic hydrocarbons could eventually offer a way to produce liquid fuels without pulling more carbon out of the ground.