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 (AE23°) 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.

Quantifying the heterogeneity of the tectonic stress field using borehole data

“…Distinct rotations of breakouts near fractures by 90° in an otherwise relatively homogeneous rock mass and stable tectonic setting have been observed by Rajabi et al . []. Similar rotations over certain well sections were documented by Davatzes and Hickman [] in the 34‐9RD2 well at Coso.…”

“…Heterogeneity of stress in boreholes has been widely observed to correlate with natural fractures and faults [e.g., Bell et al, 1992;Shamir and Zoback, 1992;Barton and Zoback, 1994;Mariucci et al, 2002;Yale, 2003, Pierdominici et al, 2011Sahara et al, 2014;McNamara et al, 2015;Rajabi et al, 2015] and lithology changes [Wileveau et al, 2007]. A self-similar scaling of stress heterogeneity observed in boreholes has been first proposed by Shamir and Zoback [1992], and Day-Lewis et al [2010] aim to link this to the magnitude-frequency distribution of earthquakes.…”

Contemporary tectonic stress pattern of the Taranaki Basin, New Zealand

“…In the studied wells, we observed localized rotation of breakouts in the vicinity of fractures and faults in the Taranaki Basin (Figure ). Such a local perturbation of the S Hmax was previously reported in the Taranaki Basin [ Camac et al ., ], and other regions worldwide, due to juxtaposition of different types of rocks, slip on active faults, and changes in the rock mechanical properties owing to high density of fractures [ Rajabi et al ., , ; Zoback , ]. It should be noted that local perturbations of breakouts in the studied wells occur in very short intervals (e.g., 10–15 m; Figure ) and do not have a significant impact on the mean S Hmax orientation.…”

“…The orientation of the S Hmax can be deviated from the regional trend, laterally and with depth, due to the presence of geological features [ Rajabi et al ., , ; Zoback , ]. The Taranaki Basin shows a significant ENE‐WSW S Hmax orientation except the west of Mount Taranaki where a number of close by indicators show rotations from the regional trend possibly owing to density contrast and magmatism as previously highlighted by Sherburn and White [].…”

Scales of Stress Heterogeneity Near Active Faults in the Santa Barbara Channel, Southern California

“…Each of these studies, and numerous others that also observed various scales of S Hmax rotations occurring with depth in individual wellbores, have attributed these rotations to the presence of faults, fractures, or density contrasts, which cause localized perturbations in the stress field. Observations from both Rajabi et al (2017) and Lin et al (2010) indicate that abrupt changes of S Hmax are more consistent with the presence of faults, while gradual rotations of S Hmax are more consistent with the presence of fractures. Unfortunately, however, there is a 75 m gap between the two breakout zones that occur in Well 01, and in the absence of borehole televiewer logs, lithological logs, or information on local scale structures near Holly, we are unable to identify the cause of this rotation.…”