NEW ZEALAND’S TARANAKI BASIN: GIANTS IN THE GRABEN?

Abstract

During the past 50 years of utilising modern techniques in New Zealand’s Taranaki Basin, explorers have both been rewarded by its bountiful accumulations and frustrated by its complicated morphology. Numerous superimposed sub-basins, depocentres, areas of uplift, interbedded volcanic edifices and recent volcanism contribute to the complexity of New Zealand’s only producing province. Exploration has been successful along the flanks of the basin, but the time has come to focus on the numerous grabens forming the Taranaki Basin.

The basin is a Late Cretaceous rift more than twice the size of the North Sea’s prolific Viking graben, but only 120 wildcats have been drilled since 1955, with only 50 offshore. Horst, tilted fault block, inversion features and thrust anticlines have been the traditional targets, but companies are showing increased interest in relatively more difficult plays involving turbiditic, volcaniclastic and diagenetic reservoirs.

The axis of the 6,000 km2 Northern Taranaki graben, the northern part of the Taranaki Basin, has not been penetrated by the drill bit and offers numerous exploration opportunities for basin floor and slope fans of Eocene and Miocene age. Acoustic scattering, diffraction and absorption within a chain of buried Miocene stratovolcanoes inhibit seismic energy from passing into the older sequences, which consist of numerous basin floor fan sequences. Long avoided by exploration programs, volcanic edifices were found to possess good reservoir characteristics and entrap hydrocarbons at Kora–1. The 7000+ km3 of layered extrusive volcanic rock in the graben cannot therefore be discounted as potential reservoir. Another play developed by Miocene magmatism is the doming of potential turbidite reservoirs by underlying igneous feeder dyke systems. In addition, the wells drilled at Kora identified a more elusive play concept—that of potentially large petroleum accumulations stratigraphically trapped downdip from diagenetically altered reservoirs, serving as sealing lithologies, near the igneous feeder dyke systems.

As most seismic records in the Northern Taranaki graben were acquired more than a decade ago, modern seismic acquisition and processing technologies are needed to penetrate the buried volcanic edifices and unlock the basin’s story. A better understanding of the basin’s complexities, more cost-effective drilling techniques and a willingness to explore for targets in the source kitchens may finally expose the true potential of the Taranaki Basin.