Influence of water hydrogen on the hydrogen stable isotope ratio of methane at low versus high temperatures of methanogenesis. (February 2019)

• Record Type: Journal Article
• Authors: Wei, Lin; Gao, Zhiye; Mastalerz, Maria; Schimmelmann, Arndt; Gao, Ling; Wang, Xin; Liu, Xiaoxue; Wang, Ye; Qiu, Zhen
• Is Part Of: Organic geochemistry. Volume 128(2019)
• Page Start: 137
• Page End: 147
• Publication Date: 2019-02
• Subjects: Methane -- Water -- Hydrogen isotope composition -- Gas generation at low temperature -- Hydrous pyrolysis; Organic geochemistry -- Periodicals; Biogeochemistry -- Periodicals; Géochimie organique -- Périodiques; 553.205
• DOI: 10.1016/j.orggeochem.2018.12.004
• Languages: English
• ISSNs: 0146-6380

Abstract

Highlights: Heating experiments <200 °C indicate <14% hydrogen transfer from water to methane. Hydrous pyrolysis experiments indicate ∼52% hydrogen transfer from water to methane. At sedimentary basin temperature most hydrogen in methane derives from organic matter. Abstract: Our laboratory experiments simulated hydrocarbon gas generation from source rocks by using low-temperature (≤200 °C) and long-term (1 month and 5 years) heating of pre-evacuated and sterilized immature shales and coals (vitrinite reflectance (VR) values of 0.21–0.47 %Ro). Source rock powders and chips were sealed in gold and Pyrex® glass tubes in the presence of waters with variable hydrogen isotopic compositions (i.e., δ 2 Hwater values of −137‰ and +1246‰) to assess the influence of water-derived hydrogen on generated gaseous hydrocarbons. In addition, hydrous pyrolysis (HP) experiments using pre-extracted shales at 330 °C and H-isotopically distinct waters were performed for comparison. The isotopic transfer of water-hydrogen to hydrocarbons generated at both low and high temperatures was quantified. Isotopic mass-balances indicate that the methane to butane (CCH4 –CC4H10 ) hydrocarbon gases from high-temperature HP experiments received an average of 53% of their hydrogen from water, whereas at low-temperatures the hydrogen transfer from water to methane ranged only from ∼1% to ∼13% of organic hydrogen. A possible explanation for differences in the extent of hydrogen transfer is that during lower-temperature hydrocarbon gas generation carbon-carbon bond breaking occurs close to the interface between minerals and organic matter, where hydrophobic organic microdomains limit the access of water. This would explain why methane generated at low temperatures mostly inherits its hydrogen from pre-existing organic hydrogen and not from water, in contrast to methane generated at higher temperatures. At higher temperatures, the physico-chemical properties of water are altered compared to those at lower temperatures. Experiments at 330 °C enhance water solubility in organic phases and facilitate stronger isotopic interactions between water and organic hydrogen. Therefore, by comparing the results obtained at low-temperature and high-temperature conditions, our laboratory experiments offer insights into the mechanisms controlling the contribution of water hydrogen to hydrocarbon gas generation in sedimentary basins.