Figure 4.

Variation in (A) gas dryness index values [C₁/(C₂+ C₃)] relative to the carbon isotope ratio of methane, (B) carbon isotope ratios of carbon dioxide relative to methane, and (C) hydrogen isotope ratios of methane relative to water. Data were collected from coalbed methane wells in the Cherokee basin (this study) and the Forest City basin (McIntosh et al., 2008). Δ¹³C-values in (B) are calculated as difference in carbon isotope ratios of carbon dioxide and methane (Δ¹³C = δ¹³C CO₂− δ¹³C CH₄). Graph (C) includes a best-fit line through Cherokee basin data (δD CH₄ = 0.05δD H₂O − 220) and lines defined by Whiticar (1999) for hydrogenotrophic and methylotrophic methanogenesis (Equations 1 and 2, respectively).