Abstract

The Great South Basin was delineated by 30,000 km of seismic data dating from the 1970s. Despite water depths averaging 750 metres, this early exploration phase resulted in 8 wells, one of which, Kawau-1A, was a 431 BCF gascondensate discovery. Other significant oil and gas shows were noted in Toroa-1, Tara-1 and Pakaha-1.

In early 2006, Crown Minerals, New Zealand's petroleum industry regulating body, conducted a new 2D seismic survey in a lightly surveyed part of the northern Great South Basin. Earlier surveys were generally recorded for five seconds, sometimes six, with up to a 3000 metre cable. The new data, acquired by CGG Multiwave's Pacific Titan, employed a 6000 metres streamer and recorded for eight seconds.

The data set was processed to pre-stack time migration by GNS Science using its access to the New Zealand Supercomputer. The processing group contended with a range of coherent noise trains. A dense set of shallow, possibly polygonal, faults, created a steeply-dipping train of interference throughout the section below and complex peg-leg multiples coincided in part with geological features. The noise was removed leaving intact the geological data they obscured.

Increasing the recording time yielded dividends by imaging, for the first time, the nature of rift faulting in the basin. Previous data showed only the tops of many fault blocks, while the new data shows a system of listric extensional faults, presumably soling out onto a mid-crust detachment. Sedimentary reflections are observed to seven seconds suggesting a thickness of up to 6000 metres of source rock units. The rotated fault blocks provide focal points for large compaction structures and the new data shows amplitude anomalies and other direct hydrocarbon indicators associated with many of these structures. The region around the Toroa-1 well was typified by anomalously low velocities, which created a vertical wipe-out zone. The new data also shows an amplitude anomaly at the well's TD which gives rise to a velocity push-down. The reservoir at Kawau-1A, known as the Kawau Sandstone, is a Late Cretaceous transgressive unit, sealed by a thick carbonate-cemented mudstone. The marine mudstone facies continues through the Paleocene, where its density decreases and carbonate cement is not present. Terrestrial deposition continued through much of the Palegene along the northwest flank of the basin, while the main part of the basin subsided to greater marine depths. In addition to the transgressive sandstone target, the basin contains sandy Paleogene facies and Tertiary turbidites may also be attractive drilling targets.

Presented at: 2007 South East Asia Petroleum Exploration Society (SEAPEX) Conference, Singapore, 2007