The migration and venting of biogenic gas in soft sediments involves a strong coupling between multiphase flow and granular mechanics. The structure of the sediment evolves as the gas migrates, and the migration pathways and mechanisms are dynamically self-selected by the competition between buoyancy, capillarity, confining stress, and the stiffness and strength of the sediment. Here, we study this process experimentally by injecting air into a quasi-2D, liquid-saturated packing of soft particles. We use high-resolution imaging to analyse the morphology of the gas as it invades, rises, and vents, and particle tracking to reveal the associated deformation field within the granular material. By systematically varying the confining stress, we show that the competition between buoyancy, capillarity, and stiffness leads to complex migration mechanisms that transition from fluidisation to pathway opening (“fracturing”) to pore invasion, with a strong and surprising impact on the dynamics of trapping and venting.