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 CH₄ and CO₂ were detected following hydrous heating of the TMS source rock in contrast to CH₄, C₂H₆, C₃H₈, and CO₂ 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 H₂S signals that are likely due to the formation of H₂S from the reaction of ES with water at 130 °C. H₂S signals correlate with a greater relative concentration of CH₄ and CO₂ compared to experiments where ES was not added, on a time-normalized basis. The correlation between the presence of H₂S and an increase in CH₄ and CO₂ concentration could indicate the occurrence of TSR. Here we propose that H₂S in siliciclastic shale can be generated in the presence of ES at low temperatures via both disproportionation of ES into H₂S and SO₄^2–, and TSR. Our findings from this study provide experimental evidence that may aid efforts to interpret the origin of H₂S in low-temperature sedimentary basins.