Interpretation of the Geochemistry of Oils of Taranaki and West Coast Region,Western New Zealand

Abstract

The predominant hydrocarbons produced in the Taranaki Basin are gas condensates, although oil has been discovered at several widespread locations and therefore remains a priority exploration objective.

Study of the oil geochemistry by means of bulk chemical characteristics, isotope and biomarker content improves our understanding of their source rocks and maturation histories. Results show that the oils and condensates throughout the region are similar in their bulk chemical character, source environment and levels of maturation suggesting a common source for all the hydrocarbons.

The source environments as indicated by biomarkers were terrestrial fresh water swamps with low bacterial anoxic conditions. The primary plant material deposited was vascular plant debris, and onshore in northern Taranaki and in the Murchison Basin, angiosperm debris was an important additional component. These angiosperm indicators are absent from the West Coast and southeastern Taranaki oils and condenstates. The overall environment of the oil sources rocks is similar to that which formed the high volatile coals of the West Coast. These coals, on source rock analyses, also reveal a perhydrous character equivalent to the high hydrogen index normally associated with marine oil source rocks.

Maturation levels of the oils, equivalent to a vitrinite reflectance level of Ro 1.0% are indicated by biomarkers. The highest maturation levels reached by drilling so far are 0.9%. suggesting that oil source rocks in Taranaki Basin are at or below the maximum drilled depth of 5.5 km.

After generation, the oils of the West Coast were slightly biodegraded as suggested by their low paraffin wax content. However, valid biomarker interpretations for source and maturation conditions are still possible.

The widespread occurrences of oil and the consistent nature of the detailed chemistry of the oils suggest that in addition to gas condensate there is a reasonable prospectivity for oil especially in and adjacent to the Central Graben are of the Taranaki Basin and in parts of the West Coast.

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