Role of pyrite during the thermal degradation of kerogen using in situ high-temperature ESR technique

Abstract

The thermal degradation of kerogen during pyrolysis in the presence and absence of pyrite as an additive was investigated and discussed from a free-radical chemistry standpoint using in situ ESR techniques. The added pyrite has pronounced effects on the activity of free radicals of kerogen samples representing the diagenesis stage. In this stage, pyrite enhances and accelerates the process of free-radical formation, where the generation of free radicals in kerogen, in the presence of added pyrite, starta at temperature lower than those observed in its absence. On the contrary, pyrite has negligible effects on the formation of free radicals in kerogen samples representing the catagenesis stage. The overall increase of free-radical activity in the presence of pyrite is consistent with the conversion of pyrite to pyrrhotite. It is suggested that pyrite acts indirectly as a catalyst through the nascent sulfur produced from the pyrite to pyrrhotite conversion process.

Geochemical evolution of organic-rich shales with increasing maturity: A STXM and TEM study of the Posidonia Shale (Lower Toarcian, northern Germany)

“…More recently, kerogen thermal degradation has been suggested to release sulphur radicals, which activity has been recognized to play an integral role in regulating the production of hydrocarbons by initiating the breaking of CeC bonds within kerogen (Lewan, 1998). The release of sulphur radicals during the Posidonia Shale kerogen degradation may thus have favored the diagenetic precipitation of pyrite, which may in turn have enhanced hydrocarbon generation processes (Bakr et al, 1991) as well as hydrothermal oxidation of hydrocarbons during bitumen maturation (Seewald, 2001a(Seewald, , 2001b.…”

Pyrite‐hydrocarbon Interaction under Hydrothermal Conditions: an Alternative Origin of H2S and Organic Sulfur Compounds in Sedimentary Environments

“…and/ or of thermochemical sulfate reduction (e.g., Powell and Macqueen, 1984;Dixon and Davidson, 1996;Zhuang et al, 1999;Machel, 2001;Worden et al, 2003;Zhu et al, 2005a;Seal Ⅱ, 2006;Basuki et al, 2008;Cai et al, 2009a;Li et al, 2012;Liu et al, 2013;Zhou et al, 2013). Additionally, it was documented that pyrite catalyzed the thermal degradation of kerogen (Bakr et al, 1991). Pure pyrite is so stable that its breakdown occurs only when the pyrolysis temperature is above 500°C (e.g., Khan, 1989;Maes et al, 1995;Gryglewicz and Jasienko, 1992).…”

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

“…Kerogens enriched in organic sulfur are commonly found in marine shales. 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.…”

“…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.…”