A GEOCHEMICAL APPRAISAL OF OIL GENERATION IN THE TARANAKI BASIN, NEW-ZEALAND

From a basin-wide evaluation of organic geochemical data, it has been possible to characterize and differentiate various source rock units a nd to establish the genetic relationships of oils. Most of the oils ar e primarily terrestrially sourced, and it is possible to recognize var ying contributions from source rocks within the Paleogene Kapuni Group and late Cretaceous Pakawau Group. This distinction is attributable t o the rise to dominance of angiosperms over gymnosperms in coastal pla in swamp communities by the Eocene. Apart from the Maui family (i.e., Maui field, Maui-4, and Moki-1) oils, the inferred relative contributi ons from the main source rock types generally correlate with the relat ive proportions of suitably thick and mature units near reservoirs, gi ven that most Cretaceous-sourced oil appears to have escaped prior to trap development. Maui family oils do appear to be primarily sourced b y Rakopi Formation (late Cretaceous) coals. In the northern part of th e Taranaki Peninsula, where heat flows are highest, Mangahewa/Kaimiro formation (Eocene) coals are the chief sources of oils. Farther south, in the Kapuni and Kupe south fields, Farewell Formation (Paleocene) c oals appear to be the main oil source rocks. Biomarkers suggest that t he onset of oil expulsion from coals occurs at a maturity level corres ponding to a vitrinite reflectance of ca. 0.8% R(0), and may be aided by the evolution of large volumes of carbon dioxide. The terrestrial i nfluence on Paleogene source rocks diminishes to the north-northwest o f the basin and increasing marine contributions to oils are observed. A late Paleocene marine black shale is the source of oil in the Kora v olcanic structure. It is possible that shales interbedded with coals, reflecting periodic marine incursions of coastal flood plains, also co ntribute to oil generation throughout much of the basin.