A Multiscale Approach towards the Characterisation of Upper Crustal Deformation at Passive Continental Margins: A Case Study on the Otway Basin, Australia
Date
2019
Authors
Burgin, Hugo Bonython
Editors
Advisors
Amrouch, Khalid
Holford, Simon
Holford, Simon
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Thesis
Citation
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Abstract
This thesis presents a multiscale structural analysis of upper crustal deformation at a passive continental margin, using the Jurassic - Quaternary Otway Basin along Australia’s southern margin as a case study. Techniques of structural analyses across the micro (calcite twin, magnetic and porefabric analyses), meso (wellbore and outcrop natural fracture analysis) and macroscales (three-dimensional seismic interpretation) providing an effective means of characterising stress and strain across space and time. The integration of these investigative methods at a passive continental margin for the first time, has assisted in reducing structural uncertainty for basin evolution models, delivering original insights into the evolution of stress within these tectonic environments. The results of this study show magnitudes of maximum differential stress as high as 69MPa during extension and continental breakup, in contrast to magnitudes as low as 13MPa during basin inversion. The influence of high extensional stresses during continental break up, resulting in layer parallel stretching (LPSt), a microstructural strain which may develop in layered rock, characterised by an azimuth of stretching or thinning, orthogonal to the orientation of regional extensional faults. LPSt occurs in the early stages of extension, prior to the development of calcite twins, natural fractures, and faults which occur progressively as the intensity and duration of extension increases. This is evidenced in the Otway Basin, where Late Cretaceous aged NE-SW and N-S oriented LPSt is co-axial with extensional azimuths during that time, derived from the stress inversion of seismic scale faults, calcite twins and natural fractures from the outcrop and wellbore. The neotectonic preservation of LPSt in the Otway Ranges, an uplifted section of Early Cretaceous sediments in the Otway Basin, suggests that early grain-scale extensional strain can be preserved during ensuing phases of inversion at continental margins. As during the process of inversion, stress is primarily released through the reactivation of previously formed extensional fault and detachment systems. A process of deformation that results in low levels of coupling between the basement and cover, an observation that is supported by the low magnitudes of compressional stress (13MPa) calculated during the same period. Additionally, the results of this study have improved our understanding of sub-surface fluid flow in the Otway Basin. Geomechanical modelling demonstrating that low contemporary magnitudes of effective normal stress, acting on NW-SE oriented faults, striking parallel to the orientation of maximum horizontal stress, results in a high risk of fault dilation. This suggests that future efforts of exploration for conventional oil and gas systems within the Otway Basin, are best focused where E-W, N-S and NE-SW striking faults interact with the major NW-SE fabric, or where the influence of basin inversion is most pronounced. A major outcome of this study is a new structural framework for the Otway Basin, one that is defined by a consistent pattern of NW-SE striking faults across much of the basin, in contrast to the previous structural model of opposing fault trends in the west and east. The new framework characterises a structural trend that is consistent with faulting patterns in sedimentary provinces to the west and east along Australia’s southern margin.
School/Discipline
Australian School of Petroleum
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, Australian School of Petroleum, 2019
Provenance
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