Chapter 9 Taranaki Basin, New Zealand
The Taranaki basin is a Cretaceous and Tertiary sedimentary basin located along the western side of the North Island, New Zealand. Initiated during the Cretaceous, the Taranaki basin lies at the southem end of a rift that developed subparallel to the Tasman Sea rift, which now separates Australia and New Zealand.
Structure of the basin has been controlled by movement along the Taranaki and Cape Egmont fault zones. Subsidence commenced in the Cretaceous and continued until the Pliocene. The predominant tectonic regime in the Taranaki basin changed from one of extension to one of compression in the early Miocene. Late Tertiary tectonics formed three primary structural types: faulted anticlines, high-angle overthrust structures, and tilted fault blocks. Kapuni and Maui gas-condensate fields are faulted anticlines; the McKee (oil) and the Ahuroa and Tariki (gas-condensate) fields are overthrust structures.
Sandstones within the Eocene Kapuni Group and the Oligocene Otaraoa Formation are the only producing reservoirs. The gas- condensate and oil are sourced from nonmarine to paralic coals and carbonaceous shales of the Late Cretaceous-Eocene Pakawau and Kapuni groups. The overlying marine sequences are organically lean and have negligible source potential.
A large proportion of the sedimentary succession was deposited during the late Tertiary. Consequently, the geothermal gradient in the Taranaki basin is moderately low «3°C/100 m). Maturation studies show that only those source rocks buried between 4000 and 4950 m are in the present oil generation and gas expulsion window.