A quantitative analysis on thermochemical sulfate reduction products of two model compounds: Implications for reaction mechanism and alteration process of hydrocarbons

• Published in: Chemical geology Vol. 661; p. 122187
• Main Authors: Sun, Zhenyu, Chen, Jian, Wang, Qiang, Jia, Wanglu, Peng, Ping'an
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 05-09-2024
• Subjects: Low-molecular-weight organic acid; Oxygen-containing compound; Quantitative analysis; Reaction mechanism; Thermochemical sulfate reduction; Low-molecular-weight organic acid; Reaction mechanism; Oxygen-containing compound; Thermochemical sulfate reduction; Quantitative analysis
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2024.122187

Thermochemical sulfate reduction (TSR) is an important redox reaction that markedly alters hydrocarbons in deep reservoirs. Although the major achievements that involve reaction mechanism, process, and geochemical characteristics were attained in the last 50 years, a quantitative TSR model that includes major products was not built, which restricts the understanding of the reaction mechanism and the generation process of hydrocarbons in TSR. In this study, two series of thermal simulation experiments of the TSR involving n-octadecane (n-C18) and n-dodecylbenzene (C12B) with MgSO4 and H2O in a confined gold-tube system were conducted, and all the major products in different phases, particularly oxygen-containing compounds, were quantitatively analyzed. The results indicated that the high-molecular-weight (HMW) oxygenated compounds in heavy products were predominated by ketones, followed by alcohol and acids, and the low-molecular-weight (LMW) organic acids in water mainly comprised acetic acids, followed by formic acid and propionic acid. Both oxygen-containing compounds prompted the TSR process but in different ways. The accumulation of HMW oxygenated compounds in the non-autocatalytic stages was essential for initiating the catalyzed reaction of TSR. In addition, the oxygenated compounds of hydrocarbons enhance polarity, making hydrocarbons easier to contact with HSO4− or [MgSO4] CIP. The LMW organic acids possibly prompted the TSR process by acidifying the water and thus facilitating the formation of HSO4− and [MgSO4] CIP. The oxidation reaction during TSR possibly converted HMW oxygenated compounds to LMW organic acids and CO2. The quantitative analysis of products in TSR revealed that TSR destroyed the hydrocarbons more rapidly but in a different way from thermal cracking and that the generation process of hydrocarbon types in TSR was in the following order: heavy hydrocarbon, gaseous hydrocarbon/light hydrocarbon/solid bitumen, and gaseous hydrocarbon/solid bitumen. Compared with oil cracking, the TSR promoted the oil reservoir to rapidly progress to more mature stages. The hydrocarbon precursor and reaction extent affected the generation process and the character of all products in the TSR. These results can provide an improved understanding of the TSR mechanism and process and be useful in the evaluation of petroleum potential for TSR-altered oils.
•The TSR generates LMW organic acids and HMW oxygenated compounds.•Oxygen-containing compounds promote the TSR process in different ways.•The oxidization and vulcanization in TSR prefer to occur in aliphatic carbons.•The TSR alters the generation process of hydrocarbons.•The TSR affects the division of hydrocarbon stages.