Petroleum microseepage anomalies over petroleum accumulations are commonly explained by rapid, vertical migration of colloidal gas bubbles through fracture networks. This article is a theoretical anal- ysis of this mechanism and of continuous gas-phase flow in frac- tures. The gas-bubble ascent mechanism is much slower than re- ported microseepage velocities, so it cannot account for observed microseepage. In contrast, continuous gas-phase flow through frac- tures can equal or exceed reported microseepage velocity, while maintaining total flux low enough so that petroleum accumulations can exist for geological lengths of time. Fracture entry pressure for bubbles is more than twice that of a continuous gas phase, so con- tinuous gas-phase migration also requires a lower pressure threshold before initiating seepage. Vertical microseepage is therefore best explained by the same mechanism interpreted for macroseepage. Although this article provides a theoretically justified mecha- nism for microseepage, it also shows why interpretation of surface microseepage signals is problematic. Fracture geometry controls seepage velocity and flux, so geochemical anomalies may indicate an increase in fracture aperture, as well as possible subsurface ac- cumulations. Larger fractures require very low gas capillary entry pressures, so in some settings, surface seepage could result from fractures over stratal migration pathways, as well as over petroleum