Method of determining the permeability of an underground medium from NMR logs of the permeability of rock fragments from the medium, based on prior calibration of the permeability resulting from this measurements, from direct measurement on rock fragments, such as cuttings for example. The method mainly comprises measuring permeability (k) from rock fragments, measuring the signal produced by a device allowing NMR analysis of the rock fragments previously saturated with a protonated liquid (brine for example), obtaining optimum values for the parameters of a relation giving the permeability as a function of the NMR signal, and applying this relation to the well zones that have already been subjected to NMR logging but for which no direct permeability measurements are available. One example of one of the applications for the present invention is the evaluation of a hydrocarbon reservoir.
