The isotopic and chemical composition of natural gases from the South Island, New Zealand

Analyses of seep gases from the prospective petroleum areas of Marlborough, Westland and Southland and of gases associated with other warm and cold springs in the South Island are presented and compared with those from Taranaki and elsewhere. Isotopic analyses of gases from the sedimentary basins show that CH4 from southern Marlborough is derived from mature sources, that from Wharf Stream being the most mature of the South Island hydrocarbon-rich gases. Three gases from the Murchison Basin show increasing maturity from W to E, with isotopic values similar to those for the McKee field. Gases from Kotuku (Westland) appear to be derived from thermogenesis at low maturity levels. Gas from the Hindley Burn (Western Southland) is probably derived from a source similar to that of the gas collected from a well drilled into the Ohai coal deposit. Gases from three areas, Marlborough, the Murchison Basin and Kotuku, have high (>5%) ethane contents and high N2/Ar ratios confirming formation from mature sources and their potential for hydrocarbon production. Gas from an alluvial delta in Lake Te Anau is produced through microbial reduction of CO2 from decomposition of buried vegetation. In contrast an unusual H2 rich gas from Fiordland contains CH4, similar in C isotopic composition to gases from greywacke, but with very depleted H isotopic composition probably due to abiogenic reduction. Several warm spring gases from fault zones are CO2 rich or N2 rich and some have microbially oxidised, isotopically enriched CH4.CH4 from Hanmer Springs and some of the other fault springs have thermogenic compositions resembling those of gases from greywackes of the axial ranges of the North Island. Ratios of 3He/4He for most gases are generally above crustal values (>O.1 RA) implying addition of mantle fluids. High values >2.5 RA are observed in CH4 rich gases discharged in the Murchison and Grey Valley basins, in Fiordland (H2 rich) and in East Otago (CO2 rich). (author). 36 refs., 2 tabs., 7 figs