Present-day stress orientation in the Clarence-Moreton Basin of New South Wales, Australia: a new high density dataset reveals local stress rotations

Abstract

Early phases of the Australian Stress Map project revealed that plate boundary forces acting on the Indo-Australian Plate control the long wavelength of the maximum horizontal present-day stress orientation in the Australian continent. However, all numerical models of the stress field to date are unable to predict the observed orientation of maximum horizontal stress in the northeast of New South Wales, Australia. Recent coal seam gas exploration in the Clarence-Moreton Basin, eastern Australia, provides an opportunity to better evaluate the state of crustal stress in this part of the continent where only limited information was available prior to this study. Herein, we conduct the first analysis of the present-day tectonic stress in the Clarence-Moreton Basin, from drilling-induced tensile fractures and borehole breakouts interpreted using 11.3 km of acoustic image logs in 27 vertical wells. A total of 2822 drilling-induced stress indicators suggest a mean orientation of N069°E (±23°) for the maximum horizontal present-day stress in the basin which is different from that predicted by published geomechanical-numerical models. In addition, we find significant localised perturbations of borehole breakouts, both spatially and with depth, that are consistent with stress variations near faults, fractures and lithological contrasts, indicating that local structures are an important source of stress in the basin. The observation that structures can have a major control on the stresses in the basin suggests that, while gravity and plate boundary forces have the major role in the long wavelength (first-order) stress pattern of the continent, local perturbations are significant and can lead to substantial changes in the orientation of the maximum horizontal present-day stress, particularly at the basin scale. These local perturbations of stress as a result of faults and fractures have important implications in borehole stability and permeability of coal seam gas reservoirs for safe and sustainable extraction of methane in this area.