A theoretical model has been developed to explain results from laboratory experiments in which reservoir cores containing oil at residual oil conditions were depressurized. In these experiments, high levels of supersaturation were obtained before gas was released from solution, with the maximum level of supersaturation increasing as the rate of depressurization increased. In addition, when gas was first released from solution it was immobile, and the gas saturation had to build up to a critical value before it could start to move through the rock matrix. This critical gas saturation value was found to increase at higher rates of depressurization. The theoretical model has two parts. The first part reproduces the experimental observation that the maximum level of supersaturation pressure attained increases as the rate of pressure reduction is increased, and also shows that the number of gas bubble nuclei formed increases as the rate of pressure reduction increases. This confirms other authors’ observations that it is necessary to postulate an increase in bubble nucleation with depressurization rate, in order to explain their experimental results. The second part of the model represents the development and mobilization of the gas phase by simulating the simultaneous growth of several gas bubble nuclei within a porous medium. It is shown that when the gas is first formed, it cannot move until one of two conditions are met; either a large enough bubble is formed by growth of a single bubble, or by several bubbles joining together, to provide sufficient buoyancy to overcome the capillary forces that restrain the bubble; or sufficient bubbles join together to form continuous channels through the matrix, through which gas can how. The model calculates the value of the gas saturation at which one of these conditions is met, and this is defined as the critical gas saturation. The theoretical model is used to examine the effect of grid size, depressurization rate, residual oil saturation, and interfacial tension on the critical gas saturation. In all cases, the results agree with the trends observed in high pressure core experiments.