Porosity of a subterranean region is estimated by accessing a nuclear magnetic resonance (NMR) relaxation distribution corresponding to NMR measurements of a subterranean region in which the NMR relaxation distribution includes multiple of peaks, fitting Gaussian functions to the NMR relaxation distribution to establish values for fitting parameters for each of the Gaussian functions, determining the porosity of the subterranean region based on the values of the fitting parameters of the Gaussian functions, and categorizing the porosity based on a relationship between the fitting parameters of the Gaussian functions and a distribution of pore sizes.