The present disclosure describes methods and systems, including computer-implemented methods, computer program products, and computer systems, for determining a permeability of a rock sample. One method includes measuring a first set of Nuclear Magnetic Resonance (NMR) relaxation times for the rock sample saturated with regular water (H2O); injecting heavy water (D2O) into the rock sample; measuring a second set of NMR relaxation times for the rock sample after injecting D2O; calculating a pore connectivity factor based on the first set of NMR relaxation times and the second set of NMR relaxation times; and calculating the permeability of the rock sample based on the pore connectivity factor.