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How does natural gas form?

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Quick answer

Natural gas forms underground when buried remains of plants and tiny marine organisms are heated and compressed over millions of years. This transforms them into kerogen, which cracks into methane and other gases. Some gas also forms at low temperatures when microbes break down organic matter in shallow sediments.

Natural gas forms when buried organic matter is heated underground over millions of years, breaking large molecules into smaller hydrocarbon gases. Most of it is methane, produced either by heat-driven cracking or by microbes in shallow sediments.

Step by step

  1. Organic matter from dead organisms settles in oxygen-poor mud and is buried by sediment.
  2. Increasing heat and pressure convert the organic matter into a waxy material called kerogen.
  3. At about 100–200°C, kerogen cracks into smaller molecules, mainly methane gas.
  4. The gas migrates upward through porous rock until it meets an impermeable layer.
  5. Trapped beneath that layer, the gas accumulates in a reservoir over millions of years.

Key things to understand

  1. Tiny marine organisms and plant matter settle in oxygen-poor mud, where they partly decay without fully breaking down.
  2. As layers of sediment pile up, heat and pressure rise, and the buried organic material becomes a waxy substance called kerogen.
  3. At roughly 100–200°C, kerogen cracks into smaller molecules, mainly methane, along with liquid oil and other gases.
  4. In shallow, cooler sediments, certain microbes also generate methane by breaking down organic matter without high heat.
  5. The gas migrates upward through porous rock until it is trapped beneath an impermeable layer, forming a reservoir.

Labelled diagram

  1. Organic matter

    Dead plankton and plants settle in oxygen-poor mud on the seafloor.

  2. Burial

    Sediment layers pile up, increasing heat and pressure on the buried material.

  3. Kerogen

    Heat transforms the organic matter into a waxy substance called kerogen.

  4. Cracking

    At 100–200°C, kerogen breaks into methane and other gases.

  5. Migration

    Gas moves upward through porous rock, seeking lower pressure.

  6. Trap

    An impermeable layer stops the gas, forming a reservoir.

How buried organic matter becomes natural gas

Real-world examples

  • The Marcellus Shale in the eastern United States holds vast natural gas reserves formed from ancient marine life.
  • The onshore Groningen field in the Netherlands, once Europe's largest gas field, supplied the continent for decades.
  • Landfills produce biogenic methane as microbes decompose food and paper waste in oxygen-poor conditions.
  • Wetlands such as the Amazon basin release methane naturally as waterlogged plants decay without oxygen.

Common mistake

✗ Myth: Natural gas is a clean, green fuel.

✓ Fact: Burning natural gas releases carbon dioxide, a greenhouse gas, though less than coal or oil per unit of energy. Methane leaks during extraction and transport also warm the planet far more than CO₂ over short periods.

People also ask

How long does natural gas take to form?
Millions of years. The process requires organic matter to be buried, heated and compressed over geological time. Some biogenic gas forms faster, in thousands of years, but commercial reserves are typically ancient.
What is natural gas made of?
It is mostly methane, a molecule of one carbon and four hydrogen atoms. It also contains smaller amounts of ethane, propane, butane and impurities such as carbon dioxide, nitrogen and hydrogen sulfide.
Where is natural gas found?
In underground rock formations called reservoirs, trapped beneath impermeable layers. Major reserves lie under the Middle East, Russia, the United States and offshore regions. It is extracted by drilling wells into these traps.
Is natural gas the same as methane?
Natural gas is mostly methane, typically 70–90%, but it also contains smaller amounts of ethane, propane, butane and impurities like carbon dioxide and hydrogen sulfide. The exact mix varies by reservoir.
How is natural gas found and extracted?
Geologists use seismic surveys to map underground rock layers, then drill wells into trapped reservoirs. The gas flows out under its own pressure or is pumped to the surface and sent through pipelines.
Why is natural gas called a fossil fuel?
It comes from the remains of living things buried millions of years ago, so it is a finite resource. Burning it releases carbon dioxide, the main greenhouse gas driving climate change.
Can natural gas form without heat?
Yes. In shallow, cooler sediments, microbes produce methane by breaking down organic matter. This biogenic gas makes up a significant share of known reserves, and the same microbial process releases methane from wetlands and landfills.

Part of the guide: Nature & Earth: Living Things and Rocks →