Dominant products and reactions during organic matter radiolysis: Implications for hydrocarbon generation of uranium-rich shales

• Published in: Marine and petroleum geology 2022-03, Vol.137, p.105497, Article 105497
• Main Authors: Wang, Wenqing, Liu, Chiyang, Liu, Wenhui, Wang, Xiaofeng, Guo, Pei, Wang, Jianqiang, Wang, Zuodong, Li, Zhongping, Zhang, Dongdong
• Format: Article
• Language: English
• Subjects: Artificial irradiation; Hydrocarbon generation; Radiation products; Structural alterations; Uranium-rich shales
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2021.105497

Organic-rich shales are generally characterized by high uranium (U) contents. The effects of U irradiation on evolution and hydrocarbon generation of organic matter (OM) have attracted substantial attention. However, details on the generation mechanisms of products and the identification of dominant reactions occurring during OM radiolysis remain understudied. In this study, γ ray was used to irradiate immature kerogen and bitumen, and electron beam was for decane. The contents and carbon (C) isotopic compositions of gaseous products, the liquid products of decane, and the functional groups alterations of kerogens and bitumen were detected. The results showed that, the product generation corresponded well to the structural alteration of OM during radiolysis. C–H and R–CH3 rupture contributed to H2 and CH4 generation, respectively. O-containing group cleavage caused CO2 generation, and O2 presence could enhance the oxidation of OM by joining in the structure of OM or by facilitating the formation of new oxides. H2O addition could not only provide O, enhancing the oxidation of OM, but could also provide H, thus promoting H2 generation. We identified cracking, cross-linking, and oxidation reactions during the OM radiolysis. C isotopic composition changes in solid OM could facilitate identification of dominant reactions. Non-closed geological environment of shales promoted the occurrence of cracking reaction. The substantial radiation energy generated from U decay in U-rich shales was significant for OM evolution via decrease in H/C, increasing O/C, and via enhancement of dealkylation, aromatization and unsaturation, which were similar to the thermal effects. Moreover, U irradiation could also promote conventional gas and immature gas generation and could provide exogenous H for hydrocarbon generation of OM, especially for OM in an over mature stage. It is essential to consider the effects of U irradiation on the evolution, hydrocarbon generation, and resources assessment of U-rich shales. •C–H, R–CH3 and O-containing bonds rupture contribute to H2, CH4 and CO2 generation.•H2O can provide exogenous H for H2 and hydrocarbons generation.•C isotopic composition changes of OM can be an indicator of dominant reaction.•Radiation like thermal effect, promotes evolution and hydrocarbon generation of OM.