Natural gas origin

For starters

Methane (CH4) can be BIOTIC or ABIOTIC.

BIOTIC methane can be microbial (generated by methanogenic microbes, Archaea) or thermogenic (generated by thermal degradation of organic matter or oil cracking). This gas is termed biotic because of its derivation from biologic compounds—mainly lipids and carbohydrates— liberated from marine and terrestrial organic matter, and/or because of the action of microbes. This is the naural gas, often associated to petroleum (oil), that we use as energy source.

ABIOTIC methane is formed by (a) geochemical reactions (gas-water-rock interactions over a wide range of formation temperatures) that do not require the presence of organic matter or microorganisms or (b) by magma degassing.

See details here.....

Etiope G. (2017). Natural Gas. Encyclopedia of Geochemistry, Earth Sciences Series, Springer, pp.1-5, doi:10.1007/978-3-319-39193-9_152-1.

See also a "geological" summary picture here