Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.   All Rights Reserved.
 

Laboratory experimental results measuring carbon isotope fractionation of the methane produced by pure cultures of methanogenic bacteria during their growth on both C₁^- (CO₂, formate, methanol, and methylamine) and C₂^- (acetate) substrates have been carried out. Isotopic effects associated with bacterial methane generation depend on the specific methane production rate, that is, the amount of methane produced by a cell or cell biomass per unit time. In some cases, the isotopic effects also depend on the specific concentration of the substrate, or the amount of bicarbonate per unit of cell biomass or per one bacterial cell.

Biogenic methane has various carbon isotopic compositions both at extremely low (d¹³C < -55) and comparatively high (d¹³C > 0) carbon isotope ratios relative to the substrate used. Theoretical calculations show that the carbon isotope content of bacterial methane can be depleted in ¹³C by -100 relative to the carbon dioxide in the culture medium.