STRUCTURAL AND STRATIGRAPHIC EVOLUTION OF THE TARANAKI BASIN, OFFSHORE NORTH ISLAND, NEW ZEALAND

Abstract

The Taranaki Basin contains the only commercial gas and condensate fields in New Zealand. Thirteen offshore wells have been drilled, three of which delineated the Maui Field while six deep tests have been drilled onshore, one of which discovered the Kapuni Field. The geology of the Taranaki Basin is synthesised into a transgressive stratigraphic framework which was modified by two tectonic phases, initial rifting and foundering, followed by wrench faulting. The basin consists of the Western Platform and Taranaki Graben Complex. The former was a relatively stable block throughout most of the Tertiary, only affected during Late Cretaceous to Eocene times by normal block faulting. The latter is bounded to the east by the Taranaki Fault Zone which was mainly active during the Miocene. To the west, the Graben Complex generally shallows across a series of en-echelon steep normal to reverse faults which often show drastic changes in throw over short distances.

Upper Cretaceous coal measures were deposited in fault angle depressions. Marine sediments were deposited in western areas by Paleocene times. A regressive phase occurred during Eocene times when coal measures were deposited in southern and eastern areas. Quartzose sandstones of these coal measures are the reservoirs in the Kapuni and Maui Fields. In Late Eocene to Oligocene times, regional submergence recommenced and mainly calcareous sediments were deposited: pelagic -rich sediments in the west, neritic limestones, sandstones and mudstones in the south and east. With the development of the Taranaki Graben Complex from the Miocene onwards, sedimentary sequences consist of graben -fill mudstones and flysch, and prograding wedges comprising the continental shelf.