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Drilling a slice of geological history - 21/10/1999

As part of a research programme on late Cenozoic shallow-marine successions in Wanganui Basin, the Institute of Geological & Nuclear Sciences (GNS) and James Cook University from Queensland Australia have drilled three deep holes behind coastal outcrops northwest of Wanganui, on the west coast of the North Island.

Led by GNS sedimentologist, Dr Tim Naish, the team spent most of September and October 1999 drilling. The holes range in depth from 100 to 200 metres, and have been fully cored, and will be electrically logged.

Geological background of the area

Studies carried out during the last 10 years have allowed a high-resolution integrated record of the basin-fill for the last 2.6 million years to be drawn together (Carter & Naish, 1999).

Pliocene-Pleistocene marine strata in Wanganui Basin comprise a 5km-thick succession of high-frequency shoreline-shelf depositional sequences. Late Pliocene-Pleistocene sequences (c. 2.0-0.4 million years ago) are continuously exposed in coastal cliffs 20km northwest of Wanganui City. They are undeformed and dip gently southwards at between 4-2° . A chronology indicates that the coastal sequences can be correlated with every sea-level cycle in the deep-sea oxygen isotope record during the last 2.6 million years. In general, major sedimentary facies within the basin fill were deposited in a range of coastal plain, shoreface, and shelf marine environments during the rise, highstand and falling part of each glacio-eustatic cycle.

Consequently, the Wanganui succession is gaining a reputation as one of the most complete shallow marine records of late Neogene sea level and climatic change in the world. This well-constrained outcrop dataset has been used as a testing ground for evaluating sequence stratigraphic models of shallow-marine depositional systems, and is an important exploration analogue for shallow-marine reservoir systems.

Aim of the drilling

High-resolution outcrop and subsurface (seismic reflection) studies of the coastal sequences have already been completed. The behind-outcrop drilling and logging comprises the third component of an integrated exploration-oriented research and teaching package. To this end it is hoped to calibrate the borehole logs with the core stratigraphy and then integrate them with the already well understood outcrop stratigraphy and seismic data. In particular, it is hoped that this integrated data set will enable accurate depositional models to be developed for shoreline-shelf sand bodies during high-frequency base-level changes.

GNS and James Cook University workers examine core at Ototoka-1

GNS and James Cook University workers examine core at Ototoka-1

Sites

The drillholes penetrate the Late Pliocene-Pleistocene succession exposed along the coastal cliffs north of Wanganui City, which comprises a 400m-thick succession of 32 superimposed unconformity-bounded shallow-marine sequences that correspond to oxygen isotope stages 100-11 (2.4 – 0.4 million years).

Castlecliff-1, on the Castlecliff beachfront, reached a total depth of 193.76m, with core recovery at 99 percent. It spudded into Stage 9 (350,000-year-old) marine sediments of the Landguard Formation. At target depth (TD), the hole had reached what is at this stage inferred to be stage 65 (1.8 million-year-old), in sediments of the Lower Maxwell Formation (formation thickness is estimated from outcrop and seismic data).

Kai-iwi-1 was plugged and abandoned on the 11th of October 1999. With an original TD of 250m, the well penetrated 5 artesian sand intervals between 75 and 126 metres and became too unstable for effective continuation of coring and wireline logging. The hole had been entirely cased to TD: 6" to 80 m, 5" PW to 120m, and reduction to HW for HQ coring at 126m. It was decided that HQ coring would have encountered more problematic sand horizons and that it was better to spud into the contingency hole, at Ototoka-1. On the positive side 95% core recovery was achieved for an interval overlapping with the base of Castlecliff-1 (0-70m). It also penetrated a new volcaniclastic section that had been removed by erosion at the coastal-base "Castlecliffian" unconformity.

After Kai-iwi-1 was abandoned, drilling moved to Ototoka-1, as it was less likely to be prone to artesian groundwater problems. The hole had a TD of 135m, penetrating the Maxwell, Tewkesbury and Nukumaruan Brown Sand Formations and finishing in the Nukumaru Limestone Formation. It is currently at 91m, although there have been some drilling difficulties with loose friable sand (in Nukumaru Brown Sand). The well is running casing, with a view to coring down to, and 5m into, the Nukumaru Limestone, a good seismic reflection marker. Once complete, Ototoka-1 will be wireline logged. The logging will move back to Castlecliff-1, where the hole will be opened out, and logged.

Ototoka-1 drill site location and rig

Ototoka-1 drill site location and rig

Recommended reading:

Carter, R.M., Naish, T.R. (editors), 1999. The high-resolution, chronostratigraphic and sequence stratigraphic record of the Plio-Pleistocene, Wanganui Basin, New Zealand. Institute of Geological and Nuclear Sciences folio series 2, version 1999.1

Contact:Greg Browne