Stable isotopes from methane and hydrogen sulfide in gas hydrates as source signatures: Influence of clay minerals, biosurfactants, and salts on isotopic selectivity

Complex physical and chemical interactions between encaged molecules and hydrate-hosting sediments during hydrate formation complicate the use of stable isotopes as source signatures. In this study, hydrates containing methane and hydrogen sulfide are formed in a pressure vessel in order to explore the effects of 1) sodium chloride (NaCl) and magnesium sulfate heptahydrate (MgSO4·7H2O), and 2) smectite clays and biosurfactants on the fractionation of stable isotopes of carbon (δ13C CH4) and hydrogen (δ2HCH4) in methane and of sulfur in hydrogen sulfide (δ34SH2S). Experiments with NaCl and MgSO4·7H2O solutions show less than 1 per mil difference in values of δ13C CH4 and up to 6.5 per mil difference in values of δ2H CH4 for free and encaged molecules. Experiments with hydrogen sulfide in similar solutions show up to 4 per mil difference in values of δ 34SH2S for free and encaged molecules, but up to 14 per mil difference between dissolved and either free molecules or encaged molecules. In experiments with methane hydrates formed from solutions in contact with smectite clays or containing biosurfactants, the difference in values of δ13CCH4 between free and encaged molecules is less than 1 per mil, whereas the difference in values of δ 2HCH4 are up to 10 per mil between free and encaged molecules. In addition, efficiency of methane consumption increased in methane hydrates formed from solutions containing biosurfactants alone or biosurfactant-smectite mixtures. In the presence of NaCl and MgSO4·7H2O salts, smectite clays, and biosurfactants, isotopic fractionation indicated by δ13CCH4 and δ2H CH4 in free and encaged molecules are small and do not complicate interpretations of gas origin. Conversely, in hydrate systems containing hydrogen sulfide molecules, values of δ34SH2S need to be interpreted with caution. Moreover, enhanced consumption of methane in hydrates formed in association with biosurfactant solutions modifies gas wetness, compromising interpretations of gas origin and thermal maturity. Small-vessel pressure experiments demonstrate unexpected complexity in fractionation of stable isotopes during formation of hydrates, complicating the interpretation of source signatures during hydrocarbon exploration and the assessment of biosignatures in other planetary bodies.