This disclosure describes systems and methods to predict viscosity of heavy oil in a geological formation, even when the geological formation also contains water such as clay-bound water, using a downhole nuclear magnetic resonance (NMR) tool. The downhole NMR tools may obtain responses include distributions of a first relaxation time T1, a second relaxation time T2, or diffusion, or a combination of these. The responses of the NMR measurements that are due to water are separated from the responses of the NMR measurements that are due to heavy oil. The responses of the NMR measurements due to heavy oil are then related to likely values of viscosity of the heavy oil based on empirical or simulated measurements.