We assessed the microbiological reduction of carbon dioxide into methane bioenergy through biochemical reaction by addition of zero valent iron (ZVI) as an alternative electron donor in oil reservoir production waters under strictly anaerobic conditions. Enhanced methane production was observed in all the treatments amended with ZVI compared with the controls. The outcome of the microbial community (Illumina Next Generation Sequencing) analysis indicated that CO2-reducing methanogens were closely related to Methanothermobacter spp. responsible for the production of methane. Moreover, the detection of FeCO3 in the culture medium amended with ZVI at the end of the experiment, characterized by X-ray Photoelectron Spectroscopy (XPS), indicated a potential CO2 transformation via mineralization under the investigation conditions with simultaneous methane production. This study offers an alternative strategy for carbon dioxide reduction into methane for energy and also a potential possibility for carbon dioxide reducing and subsequently clean bioenergy recovery in oil reservoirs. (C) 2018 Elsevier Ltd. All rights reserved.