Abstract

Hydrocarbon distributions, carbon and sulphur isotope data, and trace metal content of oils from the Taranaki Basin have been evaluated for information on source rock composition, depositional environment and maturity. Comparison has also been made of biomarker distributions in oils and potential source rocks. The data indicate that most oils are sourced mainly from terrestrial higher plant material, with only minor marine phytoplankton contributions. The source rocks appear to be coaly sediments (coals and shales) associated with coastal plain swamps of the Late Cretaceous (Rakopi Formation) and Eocene (Kapuni Group), and the latter may be generating, oil at the present in most of the eastern part of the Basin. The hydrogen-rich character of the coaly sediments is attributable to the high aliphatic content of abundant desmocollinite, which may be primarily derived from leaf cuticular membranes and bacterial remains. Gymnosperms, particularly podocarps, were the chief members of the swamp flora during the Late Cretaceous, and contribute significant quantities of characteristic diterpanes, often dominated by isopimarane. During, the Eocene angiosperms became more important, their contribution being characterised by various triterpanes, particularly 18alph(H)-oleanane and its C 24 A-ring degraded counterpart. Secondary migration distances seem limited and differences in biomarker distributions in oils appear to be directly related to compositional variations in the corresponding source rock units in a particular area. These variations are associated with changes in the composition of plant communities and suggest differing relative contributions from the Rakopi and Kapuni source rocks.

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