Natural gas is mostly methane, a simple molecule made of one carbon atom and four hydrogen atoms. As it travels through pipelines, natural gas typically contains about 90% methane, 8% ethane, and 2% propane by volume. But what actually creates this gas in the first place involves processes that range from ancient geology unfolding over millions of years to microbes quietly digesting organic matter underground.
What Natural Gas Is Made Of
Methane dominates, but raw natural gas straight from the ground is messier than what reaches your stove. Unprocessed gas contains heavier hydrocarbons like butane alongside significant amounts of carbon dioxide and hydrogen sulfide, a toxic, foul-smelling compound. Before natural gas can be sold commercially, hydrogen sulfide must be stripped down to just 4 parts per million, and carbon dioxide to less than 1%.
The cleaning process, called “sweetening,” runs the raw gas through chemical solutions that react with and absorb the unwanted compounds. The cleaned gas exits as “sweet gas” and moves on to separation units where ethane, propane, and butane are pulled out for their own uses (propane tanks, petrochemical feedstocks). What remains is pipeline-quality methane.
How Nature Produces Methane Underground
Two distinct processes generate nearly all the natural gas found in the earth. The first is biological. Microorganisms called methanogenic archaea, ancient single-celled organisms that thrive without oxygen, consume organic material buried in sediments and produce methane as a byproduct. This biogenic methane forms at relatively low temperatures, generally below 80°C (176°F), in environments like shallow sediments, swamps, and coal seams.
The second process is thermogenic, and it’s responsible for most of the natural gas we extract commercially. When organic-rich rock gets buried deep enough, the heat and pressure of the earth’s crust begin breaking down large organic molecules into smaller ones, including methane. This thermal cracking typically kicks in above 60°C but ramps up significantly at temperatures beyond 150°C (300°F), which corresponds to burial depths of several kilometers. The deeper and hotter the burial, the more completely the organic material converts to gas rather than oil.
Where the Organic Material Comes From
Natural gas traces back to organisms that lived millions of years ago. The type of original organic matter determines whether a source rock tends to produce oil, gas, or both. Geologists classify this buried organic material, called kerogen, into types based on its origin. Type III kerogen, derived primarily from terrestrial plant material like wood and leaves, is considered “gas prone” and generates natural gas preferentially when heated. Type II kerogen, sourced from marine organisms like plankton and algae, can produce both oil and gas depending on how deeply it’s been buried.
The process from living organism to extractable gas takes an enormous amount of time. Organic matter accumulates in oxygen-poor environments (seafloors, lake beds, swamps) where it doesn’t fully decompose. Layer after layer of sediment buries it deeper. Over tens of millions of years, increasing temperature and pressure transform it first into kerogen, then progressively into oil and gas. The entire sequence, from deposition through burial to gas generation, can span 50 to 300 million years.
Why Gas Stays Trapped Instead of Escaping
Once formed, natural gas is lighter than the surrounding rock fluids and migrates upward through tiny pore spaces and fractures. For a gas deposit to exist, something has to stop that upward movement. Geologists call these barriers “traps,” and they come in several forms.
Structural traps form when rock layers fold or fault into shapes that seal gas beneath them. An anticline, where rock layers arch upward like an inverted bowl, is the classic example: gas rises to the peak and can’t escape through the impermeable cap rock above. Salt domes create another type of structural trap when massive columns of salt push up through surrounding sediments, deforming rock layers and creating sealed pockets along their flanks.
Stratigraphic traps work differently. They form when changes in rock type create natural seals. A porous sandstone layer that pinches out against impermeable shale, for instance, can hold gas without any folding or faulting. In practice, many real-world gas reservoirs involve a combination of structural and stratigraphic features, and classifying a trap as purely one type or the other is often a judgment call.
Unconventional Gas: Shale, Tight Rock, and Coal
Conventional gas accumulates in porous reservoir rock after migrating from its source. Unconventional gas never made that journey. It remains locked in the same rock where it formed, or very close to it, in formations with extremely low porosity and permeability.
Shale gas sits trapped within fine-grained shale, the very rock that generated it. Coal bed methane is adsorbed onto the surface of coal, held in place by underground water pressure. Tight gas occupies sandstone or limestone with pore spaces so small that gas can barely flow through them. All three require specialized extraction techniques, most notably hydraulic fracturing, to create artificial pathways for the gas to flow toward a well.
These three gas types frequently coexist in the same geological formation. In coal-bearing strata, coal seams, organic-rich shales, and interbedded tight sandstones can all contain gas simultaneously, with natural fractures allowing some exchange between the different layers. The gas migrates short distances as burial depth, temperature, and pressure shift over geological time, creating a dynamic system where multiple gas types overlap.
Renewable Natural Gas From Modern Waste
Humans can also make natural gas without waiting millions of years. Renewable natural gas, sometimes called biomethane, is produced when bacteria break down organic waste in oxygen-free conditions. The same basic biology that creates biogenic methane deep underground operates in landfills, manure ponds, and wastewater treatment plants.
Landfills are the most familiar source. As buried garbage decomposes without oxygen, it naturally generates a gas mixture rich in methane and carbon dioxide. Many landfills capture this gas and either burn it to generate electricity or refine it further into pipeline-quality renewable natural gas. The refining process removes moisture, carbon dioxide, nitrogen, hydrogen sulfide, and trace contaminants like siloxanes.
Anaerobic digesters offer a more controlled approach. These sealed tanks are fed organic material (food waste, crop residue, livestock manure) and maintained at conditions that optimize bacterial methane production. The resulting biogas is chemically identical to fossil natural gas once cleaned, and it can be injected directly into existing pipeline networks. A third method, gasification, uses high heat to convert solid biomass into a gas mixture that can be further refined into methane.