FACTORS CONTROLLING THE CARBON ISOTOPIC COMPOSITION OF METHANE AND CARBON DIOXIDE IN NEW ZEALAND GEOTHERMAL AND NATURAL GASES

Gases from Taupo Volcanic Zone (TVZ) and Taranaki Natural gas fields of New Zealand show both similarities and differences.  The mantle helium signal is strong in both fields. In Taranaki, CH₄ generally predominates while in TVZ, CO₂ predominates and CO₂, CH₄, H₂ and N_2c concentrations relative to mantle-derived ³He are correlated with B/Cl ratios.  In the high B/Cl areas of TVZ d¹³C(CH₄) values concentrate in the -25‰ to -28‰ range, possibly from high temperature decomposition of kerogen organics, whereas in the lower B/Cl areas other processes result in d¹³C(CH₄) values of -15‰ to -35‰.

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.