Source rock characterization of the organic rich intervals of the Taranaki Basin, Offshore New Zealand
Amansure, Giovanni Ricardo
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The Taranaki Basin is a large (ca. 330,000 km²) sedimentary basin found along the west coast of the northern island of New Zealand. The basin lies partly onshore but mostly offshore below the broad continental shelf to the west of central North Island. The Taranaki Basin is the first sedimentary basin to be explored in New Zealand and is currently New Zealand’s only hydrocarbon producing basin, with approximately 418 million barrels (MMbbl) of oil and 6190 billion cubic feet (bcf) of gas produced by the end of 2011. Most of New Zealand’s known oil and gas accumulations are geochemically typed to coaly facies of Late Cretaceous and Paleogene ages. The main objective of this thesis is to characterize the source rock quality of the organic rich intervals of the Taranaki Basin, namely, the Wainui Member of the North Cape Formation and the Rakopi Formation. The Rakopi Formation comprises terrestrially deposited coal measures, while the North Cape Formation is generally composed of marine rocks. These Formations make up the Pakawau Group. The objective will be achieved using two key methods. Firstly, the derivation of TOC logs using Passey’s log overlay method (Passey et al., 1990) and secondly, the generation of source rock quality maps (i.e. source rock richness mapping and source potential index mapping). This will integrate concepts relating to petrophysical wireline logs, seismic interpretation, core log information, geochemical analysis, depth mapping and isopach mapping. The results obtained from this study confirms the petroleum potential of the organic rich intervals of the Taranaki Basin. Using Passey’s method it was shown that excellent average percent TOC values are encountered for both the Wainui Member of the North Cape Formation and the Rakopi Formation. From source potential index mapping, it can be concluded that the Rakopi formation has a high source potential index (>1000SPI) on the continental shelf, which indicates that it has excellent potential for petroleum generation. The Wainui Member however, shows less potential for petroleum generation on the shelf, this being attributed to generally low net thicknesses on the shelf.