Deformation of a hydrocarbon reservoir can ideally be predicted by estimating the effective stress acting on it. The effective stress is the difference between the total stress and a fraction (effective stress coefficient) of pore pressure. The effective stress coefficient is thus relevant for studying reservoir deformation and for evaluating 4-D seismic. The effective stress coefficient, α, of chalk from the reservoir zone of the Valhall field, North Sea was estimated from mechanical tests. The coefficient α was compared to Biot's coefficient, β (the dynamic effective stress coefficient) as calculated from density and sonic velocity data. The parameter α is highly relevant for effective stress prediction, as it is directly related to mechanical strain, whereas β is easy to estimate from logging data. Unexpectedly, our data indicate that β measured prior to deformation forms an upper rather than a lower bound for α, so that α cannot be assumed equal to one. During elastic deformation caused by pore pressure drawdown, β predicts α well.