In the Steam-Assisted Gravity Drainage (SAGD) thermal recovery process, high pressure and high temperature saturated-steam is injected into a bitumen-bearing oil sands formation. For most operations, the steam temperature ranges from about 200 to 260°C and thus under these conditions, the bitumen, in the presence of high temperature steam condensate, undergoes hydrous pyrolysis, i.e. aquathermolysis, yielding acid gases such as hydrogen sulfide and carbon dioxide. Current SAGD thermal reservoir simulation models in the literature often take into account complex spatial heterogeneity of the geology and oil composition and the physics of heat transfer, multiphase flow, gas solubility effects, and viscosity variations with temperature, however, few have taken the chemistry of SAGD into account. Here, we have added aquathermolysis reactions to thermal reservoir simulation model to understand reactive zones in the SAGD process and how the process generates acid gases via aquathermolysis. Given the requirement to constrain or handle sulfur emissions from thermal recovery processes, it is necessary to understand both the physical and chemical sides of the processes. Here, we have explored the possibility of triggering the Claus process underground for in situ scavenging of hydrogen sulfide during SAGD. The application of the research results is specifically to SAGD although the results could be extended to Cyclic Steam Stimulation as well. The results demonstrate that SAGD is not only a physical process that operates largely under gravity drainage but that it is also a chemically reactive process which generates hydrogen sulfide and carbon dioxide. The results also demonstrate that hydrogen sulfide generation reactions occur where there is sufficient heat, water, and oil and thus, the reactive zones are mainly at the edges of the steam chamber and in the liquid pool that sits above the production well. Injecting very small amount of sulfur dixode along with steam could result in initiation of Claus reaction underground resulting into conversion of hydrogen sulfide into liquid sulfur. The results of this study are significant given regulated emission limits of hydrogen sulfide from SAGD operations in Alberta, Canada, and moreover, the ability to potentially reduce emissions by altering the operating strategy or through in situ hydrogen sulfide scavengers offers an elegant way to meet these regulations.