Inconsistencies in organosulfur compounds during natural and artificial maturation

Abstract

Hydrous pyrolysis techniques are valuable for understanding and quantifying petroleum yield, thermal maturity, compositional variations, and kinetics across different kerogen types. Such data is essential for basin modeling and well planning. Previous studies have shown that sulfur-rich Type II-S kerogen generates hydrocarbons at lower temperatures due to its lower activation energy, resulting in early cracking. Consequently, organosulfur presents a challenge in numerous basins worldwide, as sulfur-rich source rocks from various regions must be examined closely to accurately predict hydrocarbon generation in each basin. Additional factors, such as total organic carbon content and mineralogy, also influence the timing and yield of oil generation. Methyldibenzothiophene isomers have proven to be reliable maturity indicators in sulfur-rich source rocks, often providing more accurate maturity predictions than biomarkers at higher maturity levels. However, few studies have closely examined the compositional and isomeric changes of alkylated methyldibenzothiophene compounds under hydrous pyrolysis conditions. In this study, hydrous pyrolysis experiments were conducted on three sulfur-rich source rocks and one sulfur-rich crude oil at temperatures between 275 and 360 °C to monitor and quantify changes in alkylated MDBT compounds as well as other aromatic hydrocarbons. The results were compared to four naturally matured Type II-S source rocks, spanning the immature to condensate windows, to evaluate the effects of hydrous pyrolysis on aromatic compounds. Significant differences in thermal maturity parameters were observed in the kerogen, bitumen, and expelled oil. While the increase in the 4-MDBT/1-MDBT ('MDR') ratio provided consistent thermal maturity estimates in natural samples, the hydrous pyrolysis experiments revealed that these geochemical markers are inconsistent under artificial thermal conditions, potentially due to additional sulfur species forming in the reactor, buffering reactions in the closed system reactor, the limited temperature range of the experiments, or other matrix-controlled interactions that are found in nature but are not occurring in the reactor. In contrast, aromatic biomarkers such as triaromatic steroids showed a positive increase in their isomer ratios relative to the experimental run temperature. The chemical differences between these organosulfur-rich samples (e.g., TOC, hydrogen index, mineralogy) suggest that the original source material—not solely the sulfur content—plays a critical role in the kinetics of alkylated dibenzothiophene compounds.