Two shallow gas-injection tests were carried out in a minor fault within the Siena Graben, Italy. Helium and carbon dioxide were injected 20 m below the ground surface through an inclined borehole intercepting the fault plane, into a sandy aquifer overlain by a clay formation. The gas-injection pressure was above hydrostatic plus capillary pressure, but below lithostatic pressure. Soil-gas, soil-exhalation flux and groundwater analyses were carried out to monitor gas breakthrough and behaviour. The experiment simulated gas rising naturally up a fault, where most of the geological boundary conditions were known. Despite its very low permeability to groundwater flow, the clay formation did not act as a complete barrier to gas rising via the fault. The gas velocity and migration pattern (namely gas channelling along the fault) were consistent with theoretical models of the migration of gas columns and gas slugs. Gas–water partitioning and the evolution of soil-gas and groundwater anomalies clearly reflected the conservative behaviour of He and the partial dissolution of CO2 into the aquifer. A general model of gas migration and behaviour is suggested which may have implications in natural resource exploration, geological storage of toxic or nuclear waste, and tectonic studies.