In a nuclear magnetic resonance measurement, the loss of coherent energy by hydrogen atoms as they interact with each other in bulk fluids. Bulk relaxation in fluids is caused primarily by fluctuating local magnetic fields arising from the random tumbling motion of neighboring molecules. Local field fluctuations may be high, but the fast movement of molecules tends to average these out. Thus the bulk relaxation depends strongly on the rate of movement and is affected by temperature and viscosity.

In water-wet rocks, hydrocarbons do not touch the pore walls and are not affected by surface relaxation. Thus the T₁ and T₂ of hydrocarbons are the result only of bulk and diffusion relaxation. This is an important feature of NMR logging. Based on this feature, direct hydrocarbon-typing techniques have been developed for the detection and characterization of hydrocarbons.