Tmax-S: A new proxy for the role of sulfur on sedimentary organic matter preservation and thermal maturation

Abstract

In this study, we introduce a new geochemical proxy based on the Tmax-S parameter presented in Cohen-Sadon et. al., (2022). We discuss Tmax-S applicability using various kerogen types and through thermal maturation experiments. Tmax-S, in analogy to the conventional Tmax (Tmax-HC), is the temperature at maximum organic-sulfur (S) elution during a Rock-Eval® 7S pyrolysis. Tmax-S demonstrates a strong dependency to the sulfide/thiophene bonds ratios. Low (< 400 °C) or high (> 415 °C) Tmax-S values indicate abundancy of thermally labile sulfide cross-linkage or refractory thiophene bonds, respectively. In sedimentary rocks, organic-S bonds distribution depends on how S is incorporated into sedimentary organic molecules (sulfurization) and its alteration during thermal maturation. Various thermally immature samples show a correlation (R² = 0.70) between Tmax-S to the ratio of reactive Fe to H₂S, indicating that reactive Fe availability controlled the sulfurization pathway through inter or intra-molecular S addition. With the organic S/C ratio, Tmax-S in immature rocks distinguish between sulfurization pathways and the overall sulfurization intensity. Therefore, Tmax-S can assess sulfurization contribution to organic matter preservation and improves the reconstruction of paleo-environmental conditions such as reactive Fe availability. During thermal maturation, Tmax-S provides a proxy for organic-S structural variations. Compared to Tmax-HC, Tmax-S has a better resolution, by a factor of six, at the early maturation stage. With the conventional Rock-Eval® proxies, Tmax-S demonstrates the dynamic of S compounds and HC generation along with the organic-S structural rearrangement in the remaining kerogen. Thus, Tmax-S is a rapid, robust and simple proxy for diagenetic and thermal maturation processes of sedimentary organic matter.