NEW THINKING ON THE PETROLEUM PROSPECTIVITY OF DEEP MESOZOIC SEDIMENTS IN NEW ZEALAND BASINS

Abstract

Geochemical studies, reinterpretation of early seismic data and information from new seismic surveys are extending the concept of economic basement for hydrocarbons in several New Zealand basins. Older Cretaceous and even Jurassic and Triassic rocks, previously considered to be 'basement' by petroleum explorationists, may have significance as petroleum prospects.

Triassic–Jurassic Murihiku Supergroup sedimentary sequences are up to 15 km thick, and the upper parts are still of low metamorphic rank. Vitrinite reflectances and Hydrogen Indexes from Murihiku Supergroup coals sampled from outcrop and drillholes indicate good oil potential, and, together with rock porosity of up to 18%, suggest that the Murihiku Supergroup may be prospective.

In the offshore Canterbury Basin, reinterpretation of seismic data has shown there is probably a thick sedimentary section below what was previously mapped as the regional basement horizon. This seismic interval can be related to a similar section developed in the adjacent Great South Basin where a mid-Cretaceous, rift- fill section of hydrocarbon-bearing rocks, drilled in the Kawau–1 well had good source and reservoir potential.

In the onshore Canterbury Basin, a recent vibroseis survey has revealed apparent sedimentary section extending down to more than 4,000 m which might also be the expression of a mid-Cretaceous rift-fill section, similar to that in the nearby Great South Basin and in the formerly adjacent Taranaki Basin, or possibly the older Murihiku Supergroup. This potential for a mature oil and gas source section provides the basis for further exploration of the area.

There are similar prospective sequences in several other New Zealand basins.