Stable isotope geochemistry of crude oils and of possible source rocks from New Zealand. 1: Carbon.
Twenty-six typical waxy oils from wells and seeps were obtained from the Taranaki, East Coast, West Coast, and Great South Basins, New Zealand. Possible source rocks (coals and shales) from all but the last basin were obtained from drill core and outcrops. The oils were processed into alkane, arom., resin, and asphaltene fractions, and ext. bitumens and kerogen were obtained from the rocks. Total oil, fractions and exts. were analyzed for their d13C values. New Zealand oils plot in the middle of the d13C range of world wide occurrences (-26 to -28 .permill.) and are similar to others of Cretaceous age. The C isotopic distributions of the arom. fraction and the isotopic type curves suggest the existence of 2 oil families: one from the East Coast, the other from Taranaki and the West Coast. Most of the type curves for New Zealand oils resemble those of unaltered terrigenous mature oils, but there are indications (canonical variable values and type curves) of different degrees of marine influence which is most pronounced in the East Coast oils. West Coast coals, 2 Nelson coals and some Maui coals show minimal isotopic differences between total oil and rock kerogen suggesting a common origin. These coals are possible source rocks, but the shales appear not to have any relation to the oils.
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