Thermal sulfate reduction by ammonium ion (NH4 (+)): implications for inorganic origin of H2S and N-2 in sedimentary basins

摘要

In this study, thermal reduction of MgSO4 by NH4Cl was operated in the presence of water at 350 A degrees C for 168 h. It was found that NH4Cl reacted with MgSO4 to produce H2S, N-2 and MgCl2 as the main products. Based on the experimental results and the assumption that the MgSO4-NH4Cl reaction could approach a state of chemical equilibrium in natural environments over geological time scales, thermochemical program HSC 6.0 was utilized to model this geochemical process occurring in sedimentary basins. The results indicate that NH4 (+) thermodynamically more easily participates in the reduction of SO4 (2-) than C-1-C-30 normal alkanes of oil and gas at reservoir temperatures. The heat liberated from the reaction of MgSO4-NH4Cl was estimated as 200.40-270.50 J/mol MgSO4 in typical oil and gas reservoirs (100-200 A degrees C), which is much smaller than previous studies. In comparison with the minimum temperature range (100-140 A degrees C) for conventional thermochemical sulfate reduction (TSR) using hydrocarbons, the inferred threshold temperature for initiating reduction of SO (4) (2-) by NH4 (+) in geological settings is only about 50-60 A degrees C. The present study shows NH4 (+) may play an additional role in TSR process by serving as an inorganic reductant, which has substantial implications for the formation of H2S and N-2 in sedimentary basins.