Oil reservoirs are complex ecosystems where microorganisms play a vital role in hydrocarbon degradation, mostly with methanogenesis as the terminal electron-accepting process. Especially in offshore oil reservoirs, sulfate-containing seawater is injected during the oil production to maintain the reservoir pressure, resulting in increased sulfate reduction in the reservoir. Methanogenesis and sulfate reduction are typically thought to be mutually exclusive because sulfate reducers outcompete methanogens thermodynamically for hydrogen and acetate. However, coexistence is evident in environments like landfills, estuaries, marine, and polluted river sediments. Here, we studied methanogenesis in an adapted sulfate-reducing microbial community, enriched from an oil reservoir, using incubation experiments with metabolic inhibitors to assess microbial activity and degradation potential. The observed methane production rate accounted for 156.9 µM/a with a parallel carbon dioxide production rate of 67 mM/a and a sulfate reduction rate of 20 mM/a, suggesting a coexistence of sulfate reduction and methanogenesis even in a well-mixed system. The microbial community was predominantly composed of potential sulfate-reducing bacteria including the genera Desulfotignum, Desulfospira, and Geoalkalibacter. In addition, fermentative bacteria such as Mesotoga and Petrotoga were abundant and the methanogenic genera Methanolobus as well as the order Candidatus Methanofastidiosales were most prevalent among the archaea. These study results suggest that even with sulfate-adapted communities and high sulfate concentrations methanogenesis can co-occur to minor extents.IMPORTANCEThis study demonstrates the coexistence of two microbial processes-methanogenesis (methane production) and sulfate reduction-in sulfate-rich environments using a microbial community derived from an oil reservoir. Conventionally, these processes were considered mutually exclusive, as sulfate-reducing microbes are thought to outcompete methanogens for shared substrates. However, this research reveals that methane production can persist alongside active sulfate reduction, challenging established paradigms of microbial competition and metabolic exclusivity.