Geochemical characteristics, depositional environment and hydrocarbon generation modeling of the upper cretaceous Pakawau group in Taranaki Basin, New Zealand

The oil, gas and condensate province of the Upper Cretaceous to Cenozoic Taranaki Basin is very important as it has become the sole commercially-producing sedimentary basin in New Zealand. An understanding of burial/thermal geo-histories of Taranaki Basin is essential for modeling hydrocarbon generation. In the present study, data from subsurface samples from selected wells have been analyzed and interpreted for characterizing source rock of the Pakawau Group along with thermal geohistory of the basin. The Upper Cretaceous Pakawau Group, made up of Rakopi (87-75 Ma) and North Cape (75-65 Ma) formations, is the eldest and most prolific organic rich hydrocarbon source rock in the basin. Their lithologies vary between carbonaceous mudstone and coal from alluvial to coastal plain depositional environments with marginal marine influence. Most samples that are interpreted contain kerogen Types II and II-III, with few samples of Type-III kerogen. This is validated by the biomarkers results, where the assessed data shows that the organic source ranges from terrestrial to marine origin. The Pakawau Group source is immature to mature, as reflected by the distribution of vitrinite reflectance (%Ro), pyrolysis T-max from pyrolysis data and biomarkers data. Vitrinite reflectance distribution shows that the Rakopi Formation is mostly within the mature oil window for hydrocarbon generation with values ranging generally between 0.5% and 0.95% Ro. Using two selected wells, the models have been interpreted to generate hydrocarbons from the Pakawau Group between Upper Paleocene and Middle Eocene. Interpretations of the burial models confirm that hydrocarbons of Pakawau Group has not yet attained peak generation and is still being expelled from the source rock to-date.