## 1 Introduction  
Recently, due to the gradual depletion of onshore oil resources and the growing global energy demand, the exploitation of offshore heavy oil resources has attracted much attention [1; 2; 3]. Bohai Bay, China, has abundant heavy oil resources with approximately 4 billion tons [4; 5]. Cyclic steam stimulation (CSS) is successfully applied to increase heavy oil recovery in heavy oil reservoirs in Bohai Bay, China [6; 7]. However, during the CSS processes, hydrogen sulfide (H\({}_{2}\)S) was detected in some heavy oil reservoirs [8]. H\({}_{2}\)S generation can even reach up to 0.044%. H\({}_{2}\)S generation from production operations is a concern in CSS recovery of offshore heavy oil because H\({}_{2}\)S poses a hazard to health, affects the integrity of wellbore tubulars, and impacts surface facility design considerations [9; 10; 11]. Therefore, it is necessary to predict H\({}_{2}\)S production prior to CSS implementation. The generation of H\({}_{2}\)S has attracted considerable attention because of its toxicity and corrosiveness. [PERSON] and [PERSON] defined the chemical reaction between oil sand and steam as an aquathermolysis reaction and considered that the C-S bond cleavage of S-containing organic compounds in oil sands was an important step in the aquathermolysis reaction [12]. [PERSON] et al. used tetrahydrothiophene as a model compound for aquathermolysis reactions, revealing the reaction path of tetrahydrothiophene cracking to H\({}_{2}\)S [13]. [PERSON] et al. proposed a kinetic model to predict the H\({}_{2}\)S generation and deduced the evolution of H\({}_{2}\)S in resins and asphalt of heavy oil [14]. [PERSON] et al. developed an aquathermolysis kinetic model for Athabasca bitumen to predict H\({}_{2}\)S concentrations, and the kinetic model was also validated against a wide range of laboratory and field data (Assabaska Oilfield, Canada) [15]. Furthermore, [PERSON] et al. conducted aquathermolysis experiments with minerals and concluded that the H\({}_{2}\)S concentration was higher when minerals were present in the aquathermolysis reaction mixture. Besides, they also reported that the reaction temperature of aquathermolysis could be reduced due to this catalytic effect [16]. [PERSON] et al. carried out aquathermolysis experiments with bitumen samples in a H\({}_{2}\)S corrosion-resistant autoclave and found that the amounts of H\({}_{2}\)S generated increased with an increase in the reaction time and temperature [17]. Meanwhile, [PERSON] et al. conducted aquathermolysis experiments of LH residual oil and reported that the use of superheated steam is effective in promoting H\({}_{2}\)S generation [18]. The aforementioned literature mainly focused on the H\({}_{2}\)S generation of onshore heavy oil. There is no concrete experimental data available, especially about the level of H\({}_{2}\)S generation during the CSS of offshore heavy oil. In addition, almost all of the aforementioned aquathermolysis tests were performed under static conditions by the reactors. The fluid displacement and the effects of reservoir rocks were not taken into account. There is still a lack of effective reaction kinetic models and numerical simulation methods to simulate H\({}_{2}\)S generation during the CSS of offshore heavy oil. In addition, numerous experiments were conducted and found that the heavy oil quality (chemical group composition, viscosity, and gas composition) was improved after aquathermolysis processes [19; 20]. However, the H\({}_{2}\)S generation during the experimental process was not investigated. Therefore, this paper presents a case study from Bohai Bay, China. First, the laboratory aquathermolysis tests were conducted to simulate the gases that are produced during CSS recovery of heavy oil. The effects of the reaction temperature and time on the H\({}_{2}\)S generation were studied. Then, a one-dimensional CSS experiment was conducted to predict H\({}_{2}\)S generation. A kinetic model for the prediction of H\({}_{2}\)S generation during the CSS of heavy oil was presented.