Subsurface Microbial Gas Formation in Geoenergy Systems: A Review of Mechanisms, Applications, and Future Perspectives

Abstract Microbial gas formation in subsurface environments governs the generation, transformation, and fate of methane (CH4), carbon dioxide (CO2), hydrogen (H2), and nitrogen (N2), with direct implications for geoenergy systems. This review integrates the metabolic pathways and reservoir-scale processes that govern microbial gas formation, including fermentation, methanogenesis, syntrophic interspecies electron transfer, denitrification, and mineral-mediated reactions. The main novelty of this review is its mechanistic-to-field perspective, linking microbial metabolism to gas-fluid behavior, oil recovery mechanisms, underground H2 storage (UHS) risks, carbon capture utilization and storage (CCUS) opportunities, and depleted reservoir reuse. For enhanced oil recovery (EOR), in situ microbial gases can support reservoir repressurization, oil swelling, viscosity reduction, and improved sweep efficiency, especially when combined with biosurfactants, acids, and biomass. In UHS, microbial activity can deplete stored H2, produce CH4 or H2S, affect gas purity, and alter permeability via biofilm growth. Microbial CO2 conversion and depleted reservoirs offer opportunities for integrated gas production and carbon management in the CCUS. Still, the performance remains constrained by reservoir heterogeneity, microbial kinetics, pressure, salinity, and nutrient delivery. The review identifies key knowledge gaps and future needs, including high-pressure experiments, real-time microbial monitoring, isotope tracing, and coupled bioreactive transport modeling. These advances are essential for predicting, controlling, and engineering microbial gas processes in low-carbon, subsurface energy systems.