Abstract: In the initial stage of production of a gas reservoir with abnormally high pressure, the forces of action of gas expansion, reservoir rock compaction and elastic expansion of compressed grains and connate water as well as water influx from the compaction of surrounding shale add to compensate for the energy loss and reduce the rate of formation pressure decline. So, the plot of shut-in bottomhole pressure (P/Z) versus cummulative production (G_p) shows two distinct segments: the initial segment with a gentle slope representing the pressure decline for the initial stage of production, followed by a steeper one for the normal pressure system, or the hydrostatic pressure system (see Figs. 1-2). Prediction of gas reserve in place based on the initial segment would certainly lead to errors; but we can not predict the gas reserve in place based on the steeper portion of the plot, for, in that case, we would have to wait till the establishment of a normal pressure system usually after the recovery of 20-25% of gas in place. Based on the princiole of energy conservation and with the application and derivation of the material balance equation, the author of the paper has proposed a method of predicting gas reserves in abnormally pressured gas reservoirs which has been proved quite dependable.