Offset Vertical Seismic Profiling in New Zealand — A Case History

The North Island of New Zealand provides the setting for the majority of land based exploration. Most recent successes have occurred in overthrust structures on the Eastern margin of the Taranaki Basin. A combination of steep dips, deep targets and elevation induced static problems provide a challenge to quantitative seismic interpretation and mapping. Notable success in the area are the Mckee field (production 8000 bopd) and the offshore Maui gas field (360 million cuft/d).

The vertical VSP has been used in the area since 1982 for prediction ahead of the bit and qualitative dip analysis. The VSP data was typically processed on the wellsite and occasionally hands-on interpretation assisted in immediate sidetrack or plug and abandon decision making. In this role the VSP had only limited success mainly due to the early limitations of the wellsite processing along with the complexity of the problem.

In 1986 and 1987 several offset VSP (OVSP) surveys were recorded to further investigate the usefulness of the VSP technique.

This paper describes how the structural interpretations of the commercial gas production field ‘Tariki’ and the accompanying ‘Ahuroa’ field were aided by the addition of VSP/OVSP data. Particular reference is made to earlier data acquired with single axis well geophones in comparison with late 1986/1987 data acquired with triaxial well geophones.

In general, the OVSP/VSP results in a high quality seismic image of pre-estimated spatial extent. A new method of interpreting OVSP data via synthetic modelling is put forward as a practical solution in areas of exceptionally high dip.