How Natural Gas Forms

Activity Source: 

Think about the energy you use every day to cook, cool your home, or travel. For most of us, the main sources of this energy are fossil fuels: coal, oil, and natural gas. Whether used directly, as gasoline, heating oil, or natural gas, or to generate electricity (by burning coal), fossil fuels are a big part of the world’s energy picture. But how do fossil fuels form?

The story starts millions of years ago, during the Carboniferous Period of the Paleozoic Era. Earth was warm and covered with plant-filled swamps and shallow seas teeming with algae and simple animal life forms such as plankton. When plants and animals died, their remains collected at the bottoms of the swamps and seas.

Some organic matter was buried under sediments before it could decay. Over millions of years, more and more sediments accumulated, and heat and pressure changed those plant and animal materials into coal, oil, and natural gas. Deposits can be trapped between layers of porous and nonporous rock. In this activity, you’ll make a model of how natural gas might be formed from decaying organic material.


  • One 1-L (1-qt) plastic bag
  • Leafy green vegetables, such as lettuce, cabbage, or spinach, at room temperature
  • Large clear measuring cup
  • Thermometer for measuring room temperature
  • Notebook
  • Pen or marker
  • Tape for sealing the bag (if needed)
  • Camera (optional)


  1. Take out your leafy green vegetables and let them come to room temperature. Tear them into pieces no larger than your hand.
  2. Add the greens to the measuring cup and pack them down as much as possible. Keep adding greens and pushing them down until the level of the greens is at the 250-mL (8-oz, or 1-c) mark.
  3. Fill the plastic bag with the greens from the measuring cup.
  4. Distribute the greens evenly along the bottom of the bag. Then roll up the bag from the bottom — to press all of the air out — and seal tightly. If the bag is not re-closable, use tape to seal the bag. This removes most of the oxygen and sets the stage for something similar to anaerobic decay, or decay in the absence of oxygen.
  5. Write a description of your greens — how they look and feel — or take a photograph of the rolled-up bag. Put the bag in a warm place, and then record the air temperature.
  6. Observe the bag each day over 10 days, and note the air temperature. Record both the temperature and the appearance of the bag each day. What happens to the greens over time? What else is happening inside the bag?
  7. When organic matter decays over time, gases are released. How is your plastic bag model similar to what happens when natural gas forms? How different?