U.S. Geological Survey Open-File Report 03-041
Figure 8

Figure 8.  Carbon isotopic composition of methane as a function of hydrocarbon gas composition, shown as gas dryness, C₁/C₂₊.  Fields are labeled to show the range of values expected for methane generated by microbial processes and thermogenic cracking reactions.  Curving lines are hypothetical mixing models that show the range of compositions that can occur due to mixing between microbial and thermogenic endmembers. The magenta curve is a hypothetical mixing line between a “Simpson-like” endmember and a “Prudhoe Gas Cap” type endmember.  The red curve is the mixing line between a very light and dry microbial gas and a less mature “Prudhoe-like” endmember.  The green curve is a mixing line between the light microbial endmember and a very wet, low thermal maturity gas.  The vertical line shows the possible effects of gas hydrate formation on the gas composition in shallow reservoirs beneath and within permafrost.