e142.
Or almost all. Note that methane from non-organic sources (‘abiogenic methane’) is discussed, though this is not a significant energy source. See, e.g., Juske Horita and Michael E. Berndt, “Abiogenic Methane Formation and Isotopic Fractionation Under Hydrothermal Conditions”, Science, 285, 5430, 1999, pp 1055–1057.