Figure 7. Methane d¹³C versus methane dD for coalbed gases in Poland and United States. Sources of data are listed in Table 1. Compositional fields modified from Whiticar et al. (1986). Composition of biogenic gases is also affected by d¹³C of carbon dioxide substrate for those gases generated by carbon dioxide reduction and by dD of associated water for those gases generated by methyl-type fermentation.