Hydrous heating experiments at 130°C yield insights into the occurrence of hydrogen sulfide and light alkanes in natural gas reservoirs

Abstract

Improved understanding of the origin of produced volatiles from conventional reservoirs and unconventional source rocks is critical for petroleum exploration and production. A series of hydrous heating experiments using two immature Type II siliciclastic source rocks, Pennsylvanian Turner Mine shale (TMS) and Devonian New Albany Shale (NAS), at 130 °C over one to two years were conducted to assess gas generation at low temperature. Elemental sulfur (ES) was added to the NAS samples to evaluate the role of sulfur on thermochemical sulfate reduction (TSR). The produced volatile composition was investigated in situ using Raman spectroscopy at the end of the heating experiments. Results show that the two source rocks yield different types and concentrations of volatiles. Only CH 4 and CO 2 were detected following hydrous heating of the TMS source rock in contrast to CH 4 , C 2 H 6 , C 3 H 8 , and CO 2 which were observed in experiments using NAS. Variations in the produced volatiles are likely the result of compositional differences within the respective source rock organic matter. Experiments involving ES show strong H 2 S signals that are likely due to the formation of H 2 S from the reaction of ES with water at 130 °C. H 2 S signals correlate with a greater relative concentration of CH 4 and CO 2 compared to experiments where ES was not added, on a time-normalized basis. The correlation between the presence of H 2 S and an increase in CH 4 and CO 2 concentration could indicate the occurrence of TSR. Here we propose that H 2 S in siliciclastic shale can be generated in the presence of ES at low temperatures via both disproportionation of ES into H 2 S and SO 4 2– , and TSR. Our findings from this study provide experimental evidence that may aid efforts to interpret the origin of H 2 S in low-temperature sedimentary basins.