Factors controlling the carbon isotopic composition of methane and carbon dioxide in New Zealand geothermal and natural gases

Abstract

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 N2 concentrations relative to mantle-derived 3He are correlated with B/Cl ratios. In the high B/Cl areas of TVZ δ13C(CH4) values are mainly in the -25 to -28range, possibly from high temperature decomposition of kerogen, whereas in the lower B/Cl areas other processes result in δ13C(CH4) values of -14 to -37‰ The relatively high heat flow in parts of the Taranaki Basin appears to have accelerated the natural gas generation process in which δ13C(CH4) is initially much more negative than the source organics (-27 to -29 but becomes more positive with increasing maturity of the source rocks. This results in a range of δ13C(CH4) values from -32 to -48‰ Isotopic equilibrium between 13CO2 and 13CH4 does not appear likely in Taranaki. In TVZ apparent isotopic temperatures are higher than reservoir temperatures but show fair correlation. This may relate both to a hydrocarbon maturity effect and to a source effect but this does not fully explain the δ13C(CH4) and δ13C(CO2) distribution within some fields. © 2004 Elsevier B.V. All rights reserved.