Australia-Banda Arc Collision and In Situ Stress in the Vulcan Sub-Basin (Timor Sea) as Revealed by Borehole Breakout Data

Abstract

Boreholes drilled in the search for oil in the Vulcan Sub-basin (Timor Sea, North West Shelf, Australia) commonly exhibit an elliptical cross-section believed to be the result of wellbore failure known as borehole breakout. Breakouts form by compressional shear failure in response to stress concentration around the borehole due to the prevailing in situ stress. The bore wall becomes elongated in the direction of least horizontal compressive stress. The orientation and shape of the breakouts are measured by the four-arm dipmeter tool. The azimuths of the long axes of breakouts in the Vulcan Sub-basin show a reasonably consistent 130?170°N trend implying that maximum horizontal compressive stress (SHmax) is oriented 040?080°N. This NE-ENE SHmax orientation in the Vulcan Sub-basin is not controlled by compression transmitted from the nearby Australia/Banda Arc collision zone. However, it is consistent with theoretical models of stress distribution in the Indo-Australian plate based on the plate-driving forces at all of its boundaries. The present SHmax orientation is consistent with either strike-slip or normal movement on the pre-existing NE-trending faults in the basin.