Natural gas forms underground from the remains of ancient plants and tiny marine organisms that died and settled into layers of sediment millions of years ago. Over time, heat and pressure transformed that buried organic material into methane, the molecule that makes up the bulk of natural gas. But not all natural gas forms the same way, and it doesn’t all come from the same type of rock. Here’s how the process works and where the gas ends up before it reaches your stove or furnace.
How Ancient Organisms Become Gas
The story starts hundreds of millions of years ago, when dead plants, algae, and plankton accumulated on ocean floors and in swamps. As layers of sediment piled on top, the organic material was buried deeper and deeper. Over geological time, increasing heat and pressure “cooked” these remains, breaking down complex organic molecules into simpler ones. The result depended on how much cooking occurred and what the starting material was: moderate heat produced oil, while higher temperatures broke oil down further into methane, the main ingredient in natural gas.
This heat-driven process is called thermogenic formation, and it happens at temperatures between roughly 157°C and 221°C, deep underground in what geologists call the “gas window.” Most of the natural gas we drill for today formed this way over tens to hundreds of millions of years, locked inside layers of heated mudstone and shale.
There’s a second, lesser-known pathway. Microorganisms called methanogens produce methane as a byproduct of digesting organic matter at much shallower depths and cooler temperatures, generally below 50°C. This biogenic gas forms closer to the surface and on far shorter timescales. Both types are chemically identical: methane is methane. The difference is how and where nature assembled it.
What’s Actually in Raw Natural Gas
Methane is the dominant component, but gas straight from a well isn’t pure. Raw “wet” natural gas typically contains ethane, propane, butanes, and pentanes, collectively known as natural gas liquids. It also carries water vapor and non-fuel impurities like carbon dioxide, nitrogen, hydrogen sulfide, and sometimes even helium. All of these need to be separated out before the gas reaches consumers. The processing involves several stages, including dehydration (removing water using chemical absorption, physical adsorption, or condensation techniques) and the removal of sulfur compounds and CO₂. What’s left after processing is mostly methane, dry and clean enough for pipeline transport.
Where the Gas Gets Trapped Underground
Once formed, natural gas migrates through rock until it either escapes to the surface or gets trapped in a geological formation. The type of trap determines how easy the gas is to reach.
Conventional reservoirs are the simplest scenario. Gas collects in porous, permeable rock (often sandstone) capped by an impermeable layer that seals it in place. These discrete pockets are relatively easy to drill into, and the gas flows out under its own pressure. Conventional production has been in use since the late 1800s.
Unconventional reservoirs are a different challenge. Instead of pooling in a single pocket, the gas is spread throughout vast, low-permeability rock formations. Three main types exist:
- Shale gas is locked in the same fine-grained mudrock where it originally formed. The rock is so dense that gas can’t flow through it on its own.
- Tight gas sits in sandstone or carbonate formations with very low permeability, requiring extra effort to extract.
- Coalbed methane is gas adsorbed onto the surface of coal seams, released when pressure in the seam drops.
Extracting gas from these unconventional sources became commercially viable with the rise of hydraulic fracturing, or fracking. The technique injects water, sand, and chemicals under high pressure into the rock, creating tiny fractures that give the trapped gas a pathway to flow toward the well. This technology is the main reason natural gas production surged in the United States over the past two decades.
Where Most Natural Gas Is Produced
The United States is the world’s largest natural gas producer, accounting for about 24% of global output in 2024. American production is concentrated in a few key regions: the Permian Basin in Texas and New Mexico, the Appalachian Basin (driven by the massive Marcellus Shale), and the Haynesville formation in Louisiana and Texas. Production held roughly steady in 2024, as gains in the Permian were offset by lower activity in Haynesville due to low prices and pipeline capacity limits in Appalachia.
Russia is the second-largest producer and saw a 7% production increase in 2024, driven by domestic consumption and exports. China’s output grew 6% as it worked to meet rising domestic demand. Norway hit a record high, up 8%, largely fueled by gas exports to the European Union. These four countries, along with other major producers like Qatar, Iran, and Canada, supply the vast majority of the world’s natural gas.
Renewable Natural Gas: A Newer Source
Not all natural gas comes from ancient underground deposits. Renewable natural gas (RNG) is produced at the surface from organic waste that’s decomposing right now. When bacteria break down manure, food scraps, sewage sludge, or landfill trash in the absence of oxygen (a process called anaerobic digestion), they produce biogas, which is primarily methane and CO₂. That biogas can be cleaned up to pipeline-quality methane, chemically indistinguishable from the fossil version.
The primary feedstocks in the United States are livestock manure (especially from dairy and swine operations), municipal solid waste in landfills (roughly 30% of which is organic material), and biosolids from wastewater treatment plants. As of recent counts, the U.S. had over 1,200 operational biogas systems at water treatment facilities, 636 at landfills, and 239 processing livestock manure. When RNG is made from manure or wastewater that would otherwise release methane into the atmosphere uncontrolled, using it as fuel can actually result in net negative greenhouse gas emissions, because it captures methane that would have escaped anyway.
RNG still represents a small fraction of total natural gas supply, but its role is growing as farms, cities, and waste facilities invest in capture systems. For the consumer, the gas works identically in pipelines, furnaces, and vehicles. The difference is where the methane originated and whether the carbon it contains is millions of years old or months old.