Viscosity of heavy oil is determined in situ in a formation by making nuclear magnetic resonance (NMR) measurements in the formation, and then calculating viscosity according to an equation of the form T 2 ⁢ LM = a + b ⁡ ( η T ) c , where T is the temperature of the heavy oil sample, η is the viscosity, T2LM is the logarithmic mean of the T2 distribution spectrum of the sample, and a, b, and c are non-zero constants. Typically, constant b has a value between 5 and 7 and constant c has a value between −0.7 and −0.5.