Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article Hydrogen donating capacity of water in catalytic and non-catalytic aquathermolysis of extra-heavy oil: Deuterium tracing study Ameen A. Al-Muntaser a,b , Mikhail A. Varfolomeev a,b, ⁎ , Muneer A. Suwaid a,b , Dmitriy A. Feoktistov c , Chengdong Yuan a,b, ⁎ , Alexander E. Klimovitskii b , Bulat I. Gareev c , Richard Djimasbe a,b , Danis K. Nurgaliev c , Sergey I. Kudryashov e , Ekaterina V. Egorova d , Artem V. Fomkin d , Oleg V. Petrashov e , Igor S. Afanasiev e , Gennady D. Fedorchenko e a Department of Petroleum Engineering, Kazan Federal University, Kazan 420008, Russia b Department of Physical Chemistry, Kazan Federal University, Kazan 420008, Russia c Institute of Geology and Petroleum Technologies, Kazan Federal University, Kazan 420008, Russia d A.P. Krylov All-Russian Oil and Gas Research Institute Moscow, 127422, Russia e Zarubezhneft Oil Company, Moscow 101000, Russia GRAPHICAL ABSTRACT ARTICLE INFO Keywords: Deuterium tracing Heavy water Donating capacity Isotope labelling In-situ deuteration Catalytic aquathermolysis ABSTRACT The goal of this work is to try to fgure out the role of water in catalytic and non-catalytic aquathermolysis by using isotope tracing techniques. For this purpose, heavy water (deuterium oxide, D 2 O) was used to replace the ordinary water (H 2 O) for catalytic and non-catalytic aquathermolysis processes of extra-heavy oil with high sulfur content in autoclave. The donating and upgrading performance of D 2 O were deeply investigated by analyzing the upgraded (deuterated) oil and their SARA (saturates, aromatics, resins and asphaltenes) fractions using diferent tracing techniques (FTIR, isotope and elemental analysis), evolved gases by GC, and change in physical-chemical properties of upgraded (deuterated) oils by viscosity measurement, SARA analysis, elemental analysis and GC, etc. The results proved the chemical role of water as a green and environmental hydrogen- donor solvent during aquathermolysis process, verifed by considerable deuterium substitution (deuteration) https://doi.org/10.1016/j.fuel.2020.118957 Received 7 March 2020; Received in revised form 6 August 2020; Accepted 10 August 2020 ⁎ Corresponding authors at: Department of Petroleum Engineering, Kazan Federal University, Kazan 420008, Russia. E-mail addresses: mikhail.varfolomeev@kpfu.ru (M.A. Varfolomeev), megycd@163.com (C. Yuan). Fuel 283 (2021) 118957 0016-2361/ © 2020 Elsevier Ltd. All rights reserved. T