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In-situ conversion of carbon dioxide into value-added products is an essential process in terms of heavy oil upgrading and utilization of the main anthropogenic greenhouse gas. In this paper, we discuss a synthesis of sodium-coated magnetite (Fe3O4) particles for in-reservoir hydrogenation of CO2. The performance of the obtained catalyst was tested in upgrading of heavy oil in a High Pressure/High Temperature (HPHT) reactor imitating the reservoir conditions during steam injection techniques. The experiments were conducted for 48 h in a CO2 environment under the steam temperature and pressure of 250 °C and 90 bar, respectively. The results showed irreversible viscosity reduction of oil from 3931 mPa.s to 2432 mPa.s after the degassing of unreacted carbon dioxide. The content of resins in the composition of upgraded oil was significantly altered from 32.1 wt% to 19.01 wt%, while the content of aromatics rose from 32.5 wt% to 48.85 wt%. The GC-MS results show the presence of alkyl benzenes and phenanthrenes, which were initially concentrated in resins and asphaltenes, in the aromatics fraction of upgraded crude oil. Thus, Na-Fe3O4 exhibits promising results for in-situ heavy oil upgrading through the hydrogenation of carbon dioxide, which contributes not only to the reduction of greenhouse gas emissions, but also enhances heavy oil recovery. [ABSTRACT FROM AUTHOR]