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-C-24 with (1) MgSO4; and (2) elemental S in sealed Au-tubes at a series of temperatures over the range 220-600 degrees C were monitored to better resolve the reaction mechanisms. Hydrogen sulfide formation from thermochemical sulfate reduction (TSR) of n-C-24 with MgSO4 was initiated at 431 degrees C, coincident with the evolution of C-2-C-5 hydrocarbons. Whereas the yields of H2S increased progressively with pyrolysis temperature, the hydrocarbon yields decreased sharply above 490 degrees C due to subsequent S consumption. Ethane and propane were initially very C-13 depleted, but became progressively heavier with pyrolysis temperature and were more C-13 enriched than the values of a control treatment conducted on just n-C-24 above 475 degrees C. TSR of MgSO4 also led to progressively higher concentrations of CO2 showing relatively low delta C-13 values, possibly due to input of isotopically light CO2 derived from gaseous hydrocarbon oxidation (e.g., more depleted CH4). Sulfur reacted with n-C-24 to produce H2S at the relatively low temperature of 250 degrees C, the H2S profile of the S treatment showed a consistent increase from 280 degrees C after a sharp increase at 250 degrees C, implicating S-hydrocarbon reactions as a potentially important source of subsurface H2S accumulations. Sulfur produced only low amounts of CO2 to 430 degrees C, indicating that abstraction of the H source for H2S occurred in the absence of C-C bond cleavages of the n-C-24 reactant. Higher yields of C-13 depleted CO2-S also showing a reactive preference for C-12 bonds-and low MW hydrocarbons were evident from 431 degrees C, although a moderate reduction (i.e., not as rapid as MgSO4-TSR) of hydrocarbon levels and increase in delta C-13 values above 490 degrees 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 (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 H2S. (C) 2011 Elsevier Ltd. All rights reserved.