A method of fabricating and utilizing an unconventional reservoir maturity chart is provided. A historical dataset for a Paleozoic or a pre-Paleozoic shale or sandstone unconventional reservoir is utilized to obtain carbon isotopic ratio values for δ13C1, δ13C2, and vitrinite reflectance equivalents (VRE). Such data is utilized to plot several maturity shapes based upon VRE values, where several of the maturity shapes have a maturity shape boundary defined by the relationship δ13C1=δ13C2. The method for utilizing the unconventional reservoir maturity chart may include determining a maturity level for the hydrocarbon gas sample based upon a relative position of the plotted data point versus a first maturity shape, a second maturity shape, a third maturity shape, and a fourth maturity shape. The method may also permit determining a production plan for the unconventional reservoir associated with the hydrocarbon gas sample.