APPEA Journal 2006—283 E. Nelson 1 , R. Hillis 1 , M. Sandiford 2 , S. Reynolds 1 and S. Mildren 3 1 Australian School of Petroleum, University of Adelaide, South Australia, 5005, Australia 2 School of Earth Sciences, University of Melbourne, Victoria, 3010, Australia 3 JRS Petroleum Research, South Australia, 5005, Australia enelson@asp.adelaide.edu.au rhillis@asp.adelaide.edu.au reynolds@asp.adelaide.edu.au mikes@unimelb.edu.au smildren@jrspet.com ABSTRACT There have been several studies, both published and unpublished, of the present-day state-of-stress of southeast Australia that address a variety of geomechanical issues related to the petroleum industry. This paper combines present-day stress data from those studies with new data to provide an overview of the present-day state-of-stress from the Otway Basin to the Gippsland Basin. This overview provides valuable baseline data for further geomechani- cal studies in southeast Australia and helps explain the regional controls on the state-of-stress in the area. Analysis of existing and new data from petroleum wells reveals broadly northwest–southeast oriented, maximum horizontal stress with an anticlockwise rotation of about 15° from the Otway Basin to the Gippsland Basin. A gen- eral increase in minimum horizontal stress magnitude from the Otway Basin towards the Gippsland Basin is also observed. The present-day state-of-stress has been interpreted as strike-slip in the South Australian (SA) Otway Basin, strike-slip trending towards reverse in the Victorian Otway Basin and borderline strike-slip/reverse in the Gippsland Basin. The present-day stress states and the orientation of the maximum horizontal stress are consistent with previously published earthquake focal mechanism solutions and the neotectonic record for the region. The consistency between measured present-day stress in the basement (from focal mechanism solutions) and the sedimentary basin cover (from petroleum well data) suggests a dominantly tectonic far-field control on the present-day stress distribution of southeast Australia. The rotation of the maximum horizontal stress and the increase in magnitude of the minimum horizontal stress from west to east across southeast Australia may be due to the relative proximity of the New Zealand segment of the plate boundary. KEYWORDS Southeast Australia, Gippsland Basin, Otway Basin, pres- ent-day stress, focal mechanism, neotectonics. INTRODUCTION To-date there is no coherent picture of the present- day state-of-stress in southeast (SE) Australia. Numerous papers have discussed the present-day state-of-stress of SE Australia either in the context of geomechanical ap- plications to petroleum exploration and development (e.g. Hillis et al, 1995; Jones et al, 2000; Dewhurst et al, 2002) or in the context of neotectonics and the tectonic origin of the present-day stress field of the area (e.g. Dickinson et al, 2002; Sandiford et al, 2004). Information on the state- of-stress is spread throughout these published papers and other unpublished reports. In the context of petroleum exploration and production, the present-day state-of-stress is significant, perhaps most notably for assessing the risk of fault reactivation (including the prediction of breached and live hydrocarbon columns) and to drilling-related issues such as wellbore stability. In the context of regional tectonics, knowledge of the pres- ent-day state-of-stress from petroleum data combined with earthquake focal mechanism solutions and the neotectonic record provide important insights into the structural and tectonic history of the region from the end-Miocene to the present-day.We have compiled published and unpublished data, and present new data, to provide the most comprehen- sive view possible of the present-day state-of-stress of SE Australia and to discuss the implications for neotectonics and the origin of stresses in the area. Present-day stress data has been used to address a va- riety of petroleum exploration and development-related geomechanical issues in SE Australian basins including: • fault reactivation and seal potential in the onshore SA Otway Basin (Penola Trough—Boult et al, 2002; Camac et al, 2004; Lyon et al, 2005); • potential CO 2 sequestration sites in the onshore Victo- rian Otway Basin (Port Campbell embayment); and, • wellbore stability/sand production and natural fracture- enhanced permeability in the offshore Gippsland Basin (Nelson and Hillis, 2005; Nelson et al, 2005), and poten- tial CO2 sequestration sites in the offshore Gippsland Basin (Root et al, 2004). It is not the intention of this paper to review in detail the geomechanical approach to the above issues, however, a short review of related literature follows. Both palaeo- and live-hydrocarbon columns have been discovered in fault-dependent traps in the Penola Trough (SA Otway Basin—Lyon et al, 2005). Brittle failure of both fault and top seal have been investigated as possible seal- breaching mechanisms to explain the occurrence of pal- aeo-columns and to predict the likelihood of trap integrity (pre-drill). • Hillis et al (1995) derived the first published present- day stress tensor (from four-arm caliper, leak-off tests and density/checkshot data) for the SA Otway Basin and summarised its implications for wellbore stability, PRESENT-DAY STATE-OF-STRESS OF SOUTHEAST AUSTRALIA