System and method for hydrocarbon reserve evaluation

During oil and gas well drilling operations, drilling fluid volume circulated per preselected vertical borehole increment drilled is continuously measured as well as the percentage by volume of hydrocarbon gas present in the return fluid. The gas is periodically sampled and analyzed to derive a relative ratio of C1 and C2 present therein. A functional relationship is then periodically computed relating the drilling fluid volume circulated per vertical increment drilled, and percentage of hydrocarbon gas therein, the rock volume of cuttings from which the gas evolved, and borehole pressure and temperature conditions at which each mud sample received respective cuttings to provide a percentage gas saturation parameter per vertical foot of reservoir. In functional response to the C1/C2 ratio magnitudes, a product is derived comprised of the gas saturation value and a scaling factor yielding MCF gas/acre-foot at borehole pressure and temperature or Bbl of oil/acre-foot, yielding a direct measurement of reserves in place.