Griffin, A.G. 2024 Structural and sedimentological interpretation of the borehole image log in well Tawatawa-1. Lower Hutt, NZ: GNS Science. GNS Science report 2024/27. 41 p.; doi: 10.21420/1MYT-9J17
Abstract
The well Tawatawa-1 was drilled in the offshore East Coast Basin in September 2004, to assess petroleum prospectivity in the basin. The well, located in 209.6 m of water, had resistivity borehole image log and wireline log data acquired through most of the well. Sidewall cores were also acquired; however, no conventional core was cut, making calibration with the image log unachievable. The image log has been interpreted and provides an insight into the subsurface structure and stress, and to a lesser extent, the sedimentological nature at the well location in the southern East Coast Basin. A simultaneous acoustic and resistivity imager device was used to acquire a resistivity image over the interval 750–1538 mRT (metres below rotary table) in Tawatawa-1. Low resistivity (i.e. conductive) fractures are the dominant fracture type in the imaged interval. In-situ stress features were also identified over the interval, particularly borehole breakouts, which provide the minimum horizontal stress direction (Shmin). Few of these features were deemed suitable in determining the present-day in-situ stress direction at the well location. Few sandstone beds were identified in the image log, however, there were not enough to confidently ascertain any sediment dispersal trends. Interbedded sediments combined with the sandstone beds suggest pulses of sediment, possibly as a series of turbidity currents being deposited at or around the vicinity of the well location. The present-day in-situ stress orientation in Tawatawa-1 suggests an Shmin azimuthal orientation of NE-SW. No indicators of SHmax were identified in Tawatawa-1, however, the inferred SHmax direction of NW-SE in Tawatawa-1 is similar to that interpreted in other wells in the southern East Coast Basin. The onshore Ranui-2 and offshore Titihaoa-1 wells also had image logs acquired in them, with NW-SE SHmax azimuthal orientations interpreted from these logs. This “fourth order of stress” (since it occurs at a wellbore scale (1–1000 m scale) SHmax orientation is approximately perpendicular to the regional-scale stress field of the southern Hikurangi Margin, which is considered to be a ‘third order’ stress orientation (<100 km). Faults have been interpreted from seismic lines in the offshore Hikurangi Margin, and it is possible that some of these faults are active and affecting the present-day stress in Tawatawa-1. The results from Tawatawa-1 can be combined with that from the nearby offshore Titihaoa-1 well, located 27 km southwest of Tawatawa-1, and onshore wells Ranui-2, Orui-1A and Te Mai-2, in the southern East Coast Basin, which have all had image logs acquired in them. Together, these data may provide an insight into the subsurface structure and stress in the southern East Coast Basin. (auth)
