Bacterial gas is mainly dry CH4 (> 99%) that has been formed from organic material at shallow depth, such as "marsh gas". It has a distinct isotope fingerprint with a δ13C between -110 and -50 (Katz, 2011). It is also referred to as "primary bacterial gas" to distinguish it from "secondary bacterial gas" which is formed after degradation of oil deposits form the degradation products (Milkov, 2010).
The generation is an anaerobic process whereby CO2 is reduced to CH4 or the transformation of acetic acid, which itself is a product of decompostion of organic matter.
Large amounts of this biogenic gas are stored in gas fields, but also in gas hydrates. The latter may well be the largest known unconventional gas resource.