Natural gas is a fossil fuel that was created eons ago by the anaerobic decomposition of organic materials. It is often found in association with oil and coal. The same types of anaerobic bacteria that produced natural gas also produce methane today.
Anaerobic bacteria are some of the oldest forms of life on earth.
They evolved before the photosynthesis of green plants released large quantities of oxygen into the atmosphere. Anaerobic decomposition occurs naturally in swamps, water-logged soils and rice fields, deep bodies of water, and in the digestive systems of termites and large animals. The primary benefits of anaerobic digestion are nutrient recycling, waste treatment, and odor control. Except in very large systems, biogas production is a highly useful but secondary benefit. The relative percentage of these gases in biogas depends on the feed material and management of the process.