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

Abstract 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 Maui gas is over an order of magnitude higher in helium concentration (up to 190 μmol mol−1) and its 3He/4He ratio of 3.8 RA (where RA=the air 3He/4He ratio of 1.4×10−6) 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 μmol mol−1. The 3He/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 3He/CO2 ratio of the Maui gases of 5 to 18×10−9 is higher than the MORB value of 0.2 to 0.5×10−9, 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 CO2/3He and CO2/C indicates δ13C(CO2) values between −7 and −5‰ close to that of MORB CO2. The remaining CO2 would appear to be mostly organically-influenced with δ13C(CO2) c.−15‰. There is some evidence of marine carbonate CO2 in the gases from the New Plymouth field. The radiogenic 4He content (Herad) varies across the Taranaki Basin with the highest Herad/C ratios occurring in the Maui field. δ13C(CH4) becomes more enriched in 13C with increasing Herad and hydrocarbon maturity. Because 3He/4He is related to the ratio of mantle to radiogenic crustal helium and 3He/C is virtually constant in the Maui field, there is a correlation between RC/RA (where RC=air-corrected 3He/4He) and δ13C(CH4) in the Maui and New Plymouth fields, with the more negative δ13C(CH4) values corresponding to high 3He/4He ratios. A correlation between 3He/4He and δ13C(CO2) was also observed in the Maui field. In the fields adjacent to Mt Taranaki (2518 m andesitic volcano), correlations of some parameters, particularly CO2/CH4, C2H6/CH4 and δ13C(CH4), are present with increasing depth of the gas reservoir and with distance from the volcanic cone.