Thermal recovery processes are well established enhanced oil recovery techniques and are widely applied for recovering heavy oil, heavy oil sands, and shale oil.

During thermal recovery the formation of hydrogen sulfide, H2S, including the H2S induced reduction of sulfate to H2S, has to be taken into account in the reservoirs. The mechanisms of H2S formation are not well understood. Therefore, a great variety of experiments was performed, mainly to investigate the formation of H2S in the presence of crude oil under variation of the temperature (250 - 320 °C) and the reaction time. Thus, the reactivity of crude oil in the presence and in the absence of H2S, as well as in the presence and in the absence of ammonium sulfate, (NH4)2SO4, or sodium sulfate, Na2SO4, was examined. An attempt is made to explain the chemical pathways leading to the formation of H2S during thermal recovery methods. The improved knowledge about the H2S-formation during thermal recovery of fossil fuels can contribute to find solutions for its prevention and must therefore be seen from the environmental standpoint of view.

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