The invention relates to a method for estimating pore pressure in subterranean shale formations such as gas- or organic-rich shale where pore pressure predictions based on either a resistivity log or a porosity-indicative log such as sonic tend to be inaccurate. The method involves combining the resistivity log with the porosity-indicative log using a Eaton formula to give an estimate of pore pressure which is accurate both for conventional water wet shale and for organic-rich shale.