Thermogenic gas

From Wikimarcellus

Thermogenic gas is gas created by the process of heating ancient organic matter over a period of millions of years under very high pressure and heat usually thousands of feet in deep in the Earth. Ancient organisms sank to the bottom of inland seas and were covered over and buried in mud. The heat and pressure reduce the organics to oil droplets and natural gas vapors that migrate into pores of rocks such as sand stone, limestone or shale.

also see Microbial gas