--> Reconciling Contemporary Stress Data With Neotectonic Structures: A Case Study From the Otway Basin, Southeastern Australia

AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Reconciling Contemporary Stress Data With Neotectonic Structures: A Case Study From the Otway Basin, Southeastern Australia

Abstract

Southeastern Australia is characterised by relatively high levels of neotectonic activity for an intraplate region. This activity comprises compressional deformation and uplift controlled by far-field plate boundary forces. Whilst the orientations of contemporary maximum horizontal stresses determined from petroleum exploration data are generally consistent with palaeostress trends inferred from neotectonic structural features, there is less consistency between stress magnitudes. The neotectonic faulting record points to a reverse fault stress regime, but studies of stress magnitudes using petroleum exploration data have mostly indicated normal or strike slip fault stress conditions. We present a new analysis of contemporary stress orientations and magnitudes in the Otway Basin, one of several basins that formed along Australia's southern margin during Cretaceous-Paleogene continental separation from Antarctica. Wellbore failure analysis of 11 wells indicates the maximum horizontal stress azimuth is ~135°N, consistent with previous studies. Lithology, underlying structural fabrics and variations in structural style with depth exert important controls on horizontal stress magnitudes. Leak-off pressures are very high (often greater than lithostatic) in post-rift marl and carbonate-dominated formations, where neotectonic deformation is typically manifested by northeast-southwest trending, low amplitude and long wavelength folds. Within the basin there is an overall increase in the minimum horizontal stress gradient of ~1–2 MPa/km from west to east. This increase corresponds with a change in structural style. In the central Otway Basin, rift-related faults are near-parallel to the maximum horizontal stress azimuth and there are comparatively low levels of neotectonic activity, whilst in the eastern Otway Basin where rift-related faults strike near-orthogonal to the maximum horizontal stress azimuth, the level of neotectonic faulting and uplift is much higher. The observation of strike-slip fault stress regimes within syn-rift sections may be due to the underestimation of horizontal stress magnitudes interpreted from leak-off pressures. Alternatively, the partitioning of stress regimes and deformation styles with depth may reflect varying mechanical properties of the basin fill. Our results show how integrating structural and geomechanical datasets can help reconcile contemporary stress data from petroleum exploration with neotectonic geological observations.