A method is provided for estimating the amount of supercharging in a formation penetrated by a wellbore. According to the method, pressure fluctuation measurements are obtained at a position in the formation accessible from the well bore and at an adjacent position in the wellbore, and a model is provided which relates variations in pressure with time at these positions to one or more adjustable parameters from which the amount of supercharging can be estimated. The or each parameter is then adjusted to optimize the fit between the pressure variations predicted by the model and the measured pressure fluctuations, and the amount of supercharging is estimated from the adjusted parameter(s).