The new release of the Australian stress map: controls on the stress pattern from the plate to field scale

Abstract

Australia is one of the most studied continents for the contemporary state of stress, due to its unusual stress pattern. The orientation of maximum horizontal stress (SHmax) in most plates, such as North America, South America and Western Europe, is primarily parallel to absolute plate motion; suggesting that the plate boundary forces that drive plate motion also control the intra-plate stress field. However, the Australian continent displays a complex pattern of stress and is not oriented parallel to its absolute plate motion. Hitherto, several 2D numerical plate scale models have revealed that the present-day stress of Australia at the first-order being controlled by plate boundary forces. However, these models were unable to reliably fit the stress pattern for eastern, central and north-eastern Australia. Although the regional (>100km wavelength) trend of SHmax in Australia is well constrained, the small (basin to well) scale variations, which are of particular importance in petroleum exploration and production, were poorly constrained. Here, we compiled more than 1000 A-D quality stress data for the continent within more than 25 stress provinces, particularly from eastern Australia and South Australia (SA). The new map shows that the SHmax orientation in NSW is generally ENE-WSW; is NE-SW in the Bowen Basin of QLD; in the Surat Basin has two trends (a major E-W and a minor NNE-SSW), and; is approximately E-W in the Cooper-Eromanga Basin of SA and QLD. SHmax is approximately E-W throughout most of SA, other than the Otway Basin (NW-SE). We compare the new compilation with previous plate scale models and show that observed SHmax orientation in most eastern basins were not predicted by published model. Furthermore, we observed numerous localised perturbations of the stress orientation, due to presence of faults, fractures and lithological contrasts; suggesting that geological structures are a key control on the stress pattern of the continent. Finally, we present a state-of-the-art finite element modelling to construct a 3D geomechanical model to predict SHmax for whole the Australian continent. The model which is subdivided into mantle, basement, sediment and ocean water; has been tested and analysed by model-independent observations. The model is promising and can predict SHmax for the whole continent. In addition, this large scale model can provide appropriate displacement or stress boundary condition for smaller reservoir-scale geomechanical models