Helium and carbon isotope systematics of natural gases from Taranaki Basin, New Zealand

The chemical and isotopic compositions of gases from hydrocarbon systems of the Taranaki Basin of New Zealand (both offshore and onshore) show wide variation. The most striking difference between the western and south-eastern groups of gases is the helium content and its isotopic ratio. In the west, the Maul gas is over an order of magnitude higher in helium concentration (up to 190 mu mol mol(-1)) and its He-3/He-4 ratio of 3.8 R-A (where R-A = the air He-3/He-4 ratio of 1.4 x 10(-6)) is approximately half that of upper mantle helium issuing from volcanic Vents of the Taupo Volcanic Zone. In the SE, the Kupe South and most Kapuni natural gases have only a minor mantle helium input of 0.03-0.32 RA and low total helium concentrations of 10-19 mu mol mol(-1). The He-3/C ratio (where C represents the total carbon in the gas phase) of the samples measured including those from a recent study of on-shore Taranaki natural gases are generally high at locations where the surface heat flow is high. The He-3/CO2 ratio of the Maui gases of 5 to 18 x 10(-9) is higher than the MORB value of 0.2 to 0.5 x 10(-9), a feature found in other continental basins such as the Pannonian and Vienna basins and in many high helium wells in the USA. Extrapolation to zero CO2/He-3 and CO2/C indicates delta C-13(CO2) values between -7 and -5 parts per thousand close to that of MORB CO2. The remaining CO2 would appear to be mostly organically-influenced with delta C-13(CO2) c.-15 parts per thousand. There is some evidence of marine carbonate CO2 in the gases from the New Plymouth held. The radiogenic He-4 content (He-rad) varies across the Taranaki Basin with the highest He-rad/C ratios occurring in the Maul field. delta C-13(CH4) becomes more enriched in C-13 with increasing He-rad and hydrocarbon maturity. Because 3He/4He is related to the ratio of mantle to radiogenic crustal helium and He-3/C is virtually constant in the Maul held, there is a correlation between R-C/R-A (where R-C = air-corrected He-3/He-4) and delta C-13(CH4) in the Maui and New Plymouth fields, with the more negative delta C-13(CH4) values corresponding to high 3He/4He ratios. A correlation between He-3/He-4 and delta C-13(CO2) was also observed in the Maul field. In the fields adjacent to Mt Taranaki (2518 m andesitic volcano), correlations of some parameters, particularly CO2/CH4, C2H6/CH4 and delta C-13(CH4), are present with increasing depth of the gas reservoir and with distance from the volcanic cone. (C) 2001 Elsevier Science Ltd. All rights reserved.