Study of thermal maturation processes of sulfur-rich source rock using compound specific sulfur isotope analysis

Thermal Transformation of Sulfur Species and Hydrogen Sulfide Formation in High-Pyrite-Containing Lacustrine Type-II Shale during Semiopen Pyrolysis

“…This stage was characterized by a noticeable increase in H 2 S yield, and the net incremental yield was 10–20 times that of the previous stage (Figure ). The generation organic sulfur-derived H 2 S is most intensive at the very beginning of kerogen decomposition. , The organic sulfur in the Chang 7 kerogen alone cannot explain such increasing generation rate of H 2 S. Therefore, contribution from organic sulfur gives way to inorganic sulfur.…”

“…At relatively low pyrolysis temperatures, thermally labile organic sulfur species, such as aliphatic sulfides, decompose to form H 2 S. As the thermal stress increases, aromatization or condensation of aromatic sulfur is also accompanied by H 2 S. − In lacustrine shales, though the organic sulfur content is much lower than that in marine shales, the pyrite content can be higher. , The transformation of pyrite to pyrrhotite and subsequently to troilite (FeS 2 → Fe 1– x S → FeS) generates active nascent sulfur radicals, which can combine with hydrogen from hydrocarbons or organic matrix to form H 2 S. − FeS 2 decomposition does not seem to accompany kerogen maturation in nature. However, under ICP conditions, where the pyrolysis temperature is higher than 300 °C, FeS 2 can be the main source of H 2 S. , In addition, H 2 S can also be generated by thermochemical sulfate reduction (TSR) reaction, in which the hydrocarbons are oxidized to CO 2 and sulfates are reduced to H 2 S. − The reaction pathways of organic sulfur and pyritic sulfur are not unidirectional. In addition to H 2 S, the released nascent sulfur can also be incorporated into organic matter or pyrite.…”

“…It is characterized by high organic carbon content (average total organic matter (TOC) is 13.81%) and extremely abundant pyrite . Its depositional environment and sulfur composition are different from the above-mentioned marine Type-II shale, , marine Type-IIS shale, − , coal, , and synthesized kerogen . In addition, the previously used open system pyrolysis, typical temperature-programmed decomposition (TPD) for coal, and hydrous pyrolysis cannot be applied to the shale’s ICP process.…”

The application of compound-specific sulfur isotopes to the oil–source rock correlation of Kurdistan petroleum

“…The organic incorporation of reduced sulfur species is also typically associated with a small isotopic fractionation (usually an enrichment; e.g., Amrani and Aizenshtat, 2004), which can vary for different diagenetic pathways. The  34 S of the organic sulfur in petroleum can be subsequently influenced by thermal maturity (Ellis et al, 2017;Rosenberg et al, 2017) as well as potentially by other common geophysical or chemical impacts on petroleum.…”

“…Alternatively, 34 S depleted H 2 S might be formed via the secondary cracking of higher MW OSCs in the bitumen, as was suggested to account for 34 S enriched OSCs produced by the hydrogen pyrolysis (HyPy) treatment of several S-rich kerogens (Grotheer et al, 2017). S isotopic trends evident from a simulated thermal maturity study of Type II-S source rocks (Rosenberg et al, 2017) included the evolution of H 2 S that was a few ‰ depleted in  34 S than the immature kerogen, consistent with a kinetic control favouring release of 32 S, although the residual kerogen and oil released showed only a minor 34 S enrichment ( 34 S increase of +0.4‰). Furthermore, the kerogen S and oil phase OSCs showed a  34 S homogenisation with increasing thermal maturity due to formation of new products from multiple S sources, thus becoming more reflective of the  34 S of the bulk kerogen (Rosenberg et al, 2017).…”