A suite of heavy oils from a single Eocene sandstone reservoir section in the Liaohe basin (northeastern China) was analyzed to study the geological controls on oil viscosity, which spans a range of almost three orders of magnitude. The heaviest oils are biodegraded, as determined from the absence of n-alkanes and the low relative amount of isoprenoid alkanes. The large variation in viscosity can be explained by mixing, to various extents, of heavy biodegraded oils with less degraded oils. This is supported by a simple mixing model, which reproduces the observed data excellently. The model may assist in predicting the viscosity of reservoired oils before production by analyzing reservoir core extracts prior to deciding which intervals to steamflood.