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We present a simple, fast, analytical expression which can be used to estimate the reservoir pressure as a function of gas saturation, for gas saturations between zero and the critical gas saturation. Use of the relation also makes it extremely easy to assess the local recovery factor as a function of pressure from the bubble point down to the pressure at which the critical gas saturation is reached. We hope that the application of this method will reduce the likelihood that operators will let the average pressures of their oil reservoirs drop to the point where significant quantities of free gas flow.