Microbial gas

From Wikimarcellus

Microbial gas is mostly methane that is emitted through decay of organic material by bacteria. It commonly occurs in landfills and dumps. It can also be found in anything ranging from septic tanks to manure storage tanks and compost heaps. It is usually found at shallow depths. It can be dissolved in ground water, or dispersed in rocks such as shale.

also see Thermogenic gas