T95 From Geochemistry of the Geosphere, Atmosphere, and Cosmos to Forensic Environmental Geochemistry II: A Tribute to Ian Kaplan

Monday, 28 October 2002
1:30 PM - 5:30 PM

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

Paper number: T95-7
Program: Technical Sessions
Time: 3:00 PM – 3:15 PM

Authors

  • John R. Hulston Presenting Author
    Institute of Geological & Nuclear Sciences, Lower Hutt, N/A

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 N2c 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.