The separate production of H 2S from the thermal reaction of hydrocarbons with magnesium sulfate and sulfur: Implications for thermal sulfate reduction

Abstract

The yields and stable C and H isotopic composition of gaseous products from the reactions of pure n-C 24 with (1) MgSO 4; 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-C 24 with MgSO 4 was initiated at 431°C, coincident with the evolution of C 2-C 5 hydrocarbons. Whereas the yields of H 2S 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-C 24 above 475°C. TSR of MgSO 4 also led to progressively higher concentrations of CO 2 showing relatively low d 13C values, possibly due to input of isotopically light CO 2 derived from gaseous hydrocarbon oxidation (e.g., more depleted CH 4).Sulfur reacted with n-C 24 to produce H 2S at the relatively low temperature of 250°C, the H 2S 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 H 2S accumulations. Sulfur produced only low amounts of CO 2 to 430°C, indicating that abstraction of the H source for H 2S occurred in the absence of C-C bond cleavages of the n-C 24 reactant. Higher yields of 13C depleted CO 2-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 MgSO 4-TSR) of hydrocarbon levels and increase in d 13C values above 490°C was attributed to their direct S reaction. This demonstrates that S, as has previously been established for MgSO 4-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 SO 4 oxidation (i.e., MgSO 4), may also account for the elemental S (S 8, S 7, S 6 and S 4) 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 H 2S. © 2011 Elsevier Ltd.