This invention is useful for determining the permeability of a geological formation using 1H NMR diffusion measurements acquired in the laboratory and using downhole 1H NMR well logging. The current technology for obtaining formation permeability downhole using NMR is not adequate for low-permeability, unconventional source rock formations with high organic content. This new method uses laboratory 1H NMR diffusion measurements for creating continuous downhole well logs of the mobile-hydrocarbon permeability of the hydrocarbon-filled pore space of downhole geological formations.