FACTORS CONTROLLING THE CARBON ISOTOPIC COMPOSITION OF METHANE AND CARBON DIOXIDE IN NEW ZEALAND GEOTHERMAL AND NATURAL GASES
The lower temperatures in Taranaki appear to have allowed time for the normal natural gas field pattern where the d13C(CH4) is initially much more negative than the source organics (-27 to -29) becoming more positive with maturity of the source rocks. This results in a range of d13C(CH4) values from -48 to -32.
Isotopic equilibrium between 13CO2 and 13CH4 does not appear likely in Taranaki. In TVZ apparent isotopic temperatures are higher than reservoir temperatures but tend to correlate to some extent. This may relate to a hydrocarbon maturity effect but this does not fully explain the correlations between d13C(CH4) and d13C(CO2) within some fields.