The separate production of H2S from the thermal reaction of hydrocarbons with magnesium sulfate and sulfur: Implications for thermal sulfate reduction

The yields and stable C and H isotopic composition of gaseous products from the reactions of pure n-C24 with (1) MgSO4; and (2) elemental S in sealed Au-tubes at a series of temperatures over the range 220–600 °C were monitored to better resolve the reaction mechanisms. Hydrogen sulfide formation from thermochemical sulfate reduction (TSR) of n-C24 with MgSO4 was initiated at 431 °C, coincident with the evolution of C2–C5 hydrocarbons. Whereas the yields of H2S increased progressively with pyrolysis temperature, the hydrocarbon yields decreased sharply above 490 °C due to subsequent S consumption. Ethane and propane were initially very 13C depleted, but became progressively heavier with pyrolysis temperature and were more 13C enriched than the values of a control treatment conducted on just n-C24 above 475 °C. TSR of MgSO4 also led to progressively higher concentrations of CO2 showing relatively low δ13C values, possibly due to input of isotopically light CO2 derived from gaseous hydrocarbon oxidation (e.g., more depleted CH4). reacted with n-C24 to produce H2S at the relatively low temperature of 250 °C, the H2S profile of the S treatment showed a consistent increase from 280 °C after a sharp increase at 250 °C, implicating S-hydrocarbon reactions as a potentially important source of subsurface H2S accumulations. Sulfur produced only low amounts of CO2 to 430 °C, indicating that abstraction of the H source for H2S occurred in the absence of C–C bond cleavages of the n-C24 reactant. Higher yields of 13C depleted CO2—S also showing a reactive preference for 12C bonds—and low MW hydrocarbons were evident from 431 °C, although a moderate reduction (i.e., not as rapid as MgSO4–TSR) of hydrocarbon levels and increase in δ13C values above 490 °C was attributed to their direct S reaction. This demonstrates that S, as has previously been established for MgSO4–TSR, has a reactive preference for hydrocarbons of high MW. The reaction of low MW hydrocarbons with the S reactant (i.e., S) or the S produced by SO4 oxidation (i.e., MgSO4), may also account for the elemental S (S8, S7, S6 and S4) and organic S products detected in the solvent extracted residue of both treatments. Field translation and validation of the molecular and stable isotopic trends identified in this laboratory study should help to resolve the relative contributions of different sources and competing processes to subsurface accumulations of H2S.