Microbial diversity and community shifts in a petroleum reservoir under production: effects of water breakthrough and anthropogenic alterations

Subsurface petroleum reservoirs host indigenous microorganisms that survive extreme conditions and long-term isolation. Microbial activity in these environments can contribute to adverse effects such as oil biodegradation and reservoir souring. Unlike the broader deep biosphere, oil reservoirs are frequently subjected to anthropogenic disturbances, particularly during production, when processes like water injection introduce external microbes and electron acceptors. In this study, we investigated microbial diversity, community structure, and the impact of water breakthrough using 16S rRNA gene and metagenomic sequencing of produced fluids, production water, and injection water samples from the Edvard Grieg oil reservoir offshore Norway. We found clear regional heterogeneity in community composition, characterized by overall low diversity, dominated by thermophilic, anaerobic, and halotolerant taxa. The southern region (wells A13, A17, A18, and A19) exhibited lower diversity, while the microbial community composition of well A07 showed a distinct signature. The prevailing genera included the strictly anaerobic bacterium Thermoanaerobacter and the hyperthermophilic archaeon Thermococcus . Water breakthrough triggered shifts in community structure, not because of widespread replacement by injected microbes, but due to the increase in sulfate-reducing bacteria. Comparison between sequence data from production fluids and water samples allowed the identification of microbial signatures that can act as cost-effective tools for monitoring oil reservoir processes and integrity.