The influence of non-hydrocarbon compounds on the thermal decomposition of kerogens and oils at geologic temperature conditions is a topic of great interest for petroleum system modeling and prospecting. Organic sulfur is thought to be an accelerant due to the facile nature of associated radical generation. However, the mechanism by which this occurs is relatively unknown, and the effects of decreasing temperature from laboratory conditions to temperatures more representative of natural systems have not been well studied. In this study, we investigate the influence of organic sulfur (diethyl disulfide, DEDS) on the decomposition of a heavy oil analog (phenyldodecane, PDD) through a combination of confined anhydrous pyrolysis experiments in gold capsules and chemical kinetic modeling. Mixtures of PDD and DEDS (~2.5 wt.% S) were reacted between temperatures of 250 to 350°C at 35 MPa under confined, single phase anhydrous conditions. PDD conversion and product speciation were analyzed by gas chromatography. We used the open-source MIT software package Reaction Mechanism Generator (RMG) to automatically construct chemical kinetic mechanisms of PDD and DEDS decomposition using elementary reactions. Quantum chemical calculations were used to refine the most important thermodynamic and kinetic parameters in the models, which are validated over several existing experimental datasets in literature.
Experimentally, we show that DEDS does not substantially accelerate PDD decomposition at 350°C (after 72h), in contrast to previous investigations in fixed volume reactors, but does exhibit statistically significant acceleration effects on decomposition at 250°C (after 1000h). The kinetic models show that DEDS is predicted to react within an hour in the presence of PDD at 350°C, forming mostly ethane and hydrogen sulfide, whereas conversion of DEDS occurs more slowly at 250°C, allowing sustained interactions with PDD. This is consistent with remnant DEDS only being detected in the products of the 250°C experiment. The timescale of available free radicals may therefore be important in controlling the pyrolysis of alkylaromatic compounds; the greater longevity of the sulfur-based free radicals at 250°C accelerates PDD decomposition. These results support the hypothesis that organic sulfur radicals likely play a key role in accelerating Type II-S kerogen maturation at geologic conditions, and may also have implications for organosulfur-rich oils.