A method for deriving at least one pore or fluid relaxation parameter and endpoint selected from the group consisting of a longitudinal T1 pore surface relaxivity constant (ρ1), a transverse T2 pore surface relaxivity constant (ρ2), a pore surface-to-volume ratio (A/V), an equivalent pore-throat radius (req), and a bulk fluid relaxation time (TB) comprising: identifying modes in NMR T1-T2 data; assigning the modes to a poro-fluid class; clustering the modes based on poro-fluid class; estimating TB based on an asymptote fit of the clusters using T1 and T2 relaxation mechanisms in a bulk fluid relaxation-dominated limit; estimating ρ2/ρ1 based on an asymptote fit of the clusters using T1 and T2 relaxation mechanisms in a surface relaxation-dominated limit; fitting the T1 and T2 relaxation mechanisms to the clusters using the estimated TB; and deriving the pore or fluid relaxation parameter and endpoint for the poro-fluid classes from the fit.