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Type: Theses
Title: Present-day crustal stress pattern across spatial scales: analysis and interpretation from plate-wide to local-scales
Author: Rajabi, Mojtaba
Issue Date: 2016
School/Discipline: Australian School of Petroleum
Abstract: This thesis analyses the present-day crustal stress field (from 0 to 40 km depth) in a wide variety of tectonic settings and spatial scales, using data compiled in Australia (an intraplate tectonic setting), New Zealand (a convergent boundary with transform faults) and Iceland (a spreading centre overlying a hot-spot). The contemporary stress patterns of 20 sedimentary basins were interpreted by the author using analysis of wellbore image and four-arm caliper logs in 750 wells, resulting in approximately 1000 new quality-ranked stress indicators from wells. This extensive dataset of borehole breakout and drilling-induced fracture data was combined with over 300 additional wellbore stress indicators compiled from published or in-press sources. In addition, other sources of stress information, such as recent earthquake data, were compiled from various seismological catalogues and quality ranked according to the World Stress Map project (WSM) guidelines. The majority of the data analysed in this thesis are from regions where there were no, or sparse, in-situ stress information in the WSM database. The first part of the thesis presents extensive wellbore data analysis of the contemporary maximum horizontal stress orientation (SHmax) in the Australian continent, which is entirely located in the Indo-Australian Plate. Previous studies on the stress field of Australia revealed that the complex stress pattern of the continent is controlled, at a first-order, by large-scale forces exerted at the plate boundaries. However, prior analysis of the contemporary Australian stress pattern has been unable to model or explain the stress pattern observed in most of eastern Australia, and has not extensively addressed the numerous smaller scale variations in stress orientation that are frequently observed in most basins. The recent development of unconventional reservoirs in Australia has resulted in a greatly increased amount of new data for stress analysis in previously unstudied or poorly-constrained areas in eastern Australia. In addition, stress analysis in conventional hydrocarbon, mineral and geothermal exploration wells in all other parts of the continent provided the opportunity to review and update the Australian Stress Map (ASM), and collect the first reliable and robust stress datasets from areas outside of sedimentary basins. The first part of this thesis presents an extensive analysis of borehole image and oriented-caliper logs in various parts of New South Wales (Gunnedah, Clarence- Moreton, Darling, Gloucester, Bowen-Surat and Sydney basins), Queensland (Bowen, Surat, Cooper-Eromanga and Galilee basins), South Australia (Officer, Cooper and Eromanga basins and mineral/geothermal wells), Western Australia (Browse Basin), Northern Territory (McArthur and Pedirka basins), Victoria and Tasmania (Gippsland, Otway, Bass and Sorell basins). In addition, all other published data for the stress analysis in the Australian continent was compiled to provide the most comprehensive stress map of Australia ever produced. This represents the first update of the ASM in 13 years, and raises the amount of stress data in the continent from 594 to 2140 data records, as well as an increase in the number of defined stress provinces to 30 (from 16 provinces in 2003). Analysis of stress provinces throughout the Australian continent reveals four major trends for the orientation of SHmax, including NE-SW in northern, northeastern Australia as well as Bonaparte and Canning Basins in northwestern Australia, E-W in most part of Western Australia and South Australia, ENE-WSW in most parts of eastern Australia and NW-SE in southeastern Australia. In addition, highdensity datasets in several sedimentary basins of eastern Australia reveal substantial stress perturbations at smaller scales (from tens of kilometres to the meter scale) owing to various geological structures, including basement structures, faults, fractures and lithological contrasts. Along with the Australian stress analysis conducted in this study, a 3D geomechanical-numerical model is constructed to predict the regional stress pattern of the continent. The model has several inputs, including various strength layers and rock mechanical data of different lithologies; and is subdivided into mantle, basement and sediment. The presented large-scale 3D model in this thesis fits the first-order stress state of the Australian continent much better than previous published models. Hence, this predictive model can provide appropriate displacements or stress boundary conditions for high-resolution basin and reservoir-scale 3D geomechanical models. The second part of this thesis presents the first comprehensive stress map of New Zealand with 652 insitu stress data records, including 183 wellbore data records from the Taranaki, East Coast and Canterbury basins. The complex interaction of the Pacific and Australian plates at New Zealand provides a unique location to study the present-day stress field in a tectonically active area. Nine stress provinces are defined across New Zealand, which highlight the dominant role of Pacific Plate subduction beneath the Indo-Australian Plate, along the Hikurangi Margin, in the stress pattern of North Island’s New Zealand. The analysis of stress provinces in the North Island revealed two regional SHmax orientations, including an E-W trend around the eastern coast (~040°S) that is perpendicular to the subduction trench, and a prevailing NE-SW SHmax orientation in the intra-arc and back-arc regions that is parallel to the trench. The SHmax orientation in much of the South Island is ESE-WNW, which is sub-parallel to the absolute motion of the Pacific Plate in this region. The stress analyses across New Zealand revealed significant inconsistencies between the SHmax orientations and major active strike-slip faults (such as Alpine and Marlborough fault systems), which further supports the prevalent hypotheses that these large active tectonic features are weak. The third part of this thesis examines the stress pattern of Iceland which has a unique geological and tectonic setting. It is located above a hotspot and on an exposed portion of the Mid-Atlantic Ridge, at the boundary of the North American and Eurasian plates. The contemporary stress pattern of Iceland has not been comprehensively investigated prior to this study. This thesis presents the contribution of the author in the first extensive compilation of in-situ stress data across Iceland. In particular, the author analysed acoustic wellbore image logs in 57 geothermal wells. The wellbore results were combined with other stress indicators, including overcoring measurements, geological information and earthquake focal mechanism solutions resulting in a dataset with 495 SHmax orientation data records. Analysis of stress data in Iceland revealed four regional SHmax orientations across the island. The SHmax is parallel to the rift axes around the active spreading centres while it is parallel to absolute plate motion in regions that are located far away from the ridge such as Westfjords. The orientation of SHmax in the northern part is NNW-SSE that changes to N-S in central part and to NE-SW in the southern Iceland. The final part of this thesis presents the contribution the author has made to the forthcoming (2016) release of the WSM project, which is a collaborative effort that has involved dozens of researchers from across the world. This year marks the 30th anniversary of the WSM project, with significant improvements in the data qualities and quantities. The new release of the WSM project contains 42870 data that is an increase of > 20,000 data records from the prior 2008 WSM release. The contribution of the author in the WSM includes the latest in-situ stress data compilations in Australia, New Zealand, Iceland and the Middle East, and represents the largest increases in borehole data stress data by any single contributor to the WSM. The resulting data increase shows stress orientation variability in several areas at regional and local-scale, which again questions one of the original findings of the WSM project, namely that the crustal present-day stress pattern is primarily controlled by plate boundary forces. This original hypothesis is again re-visited herein, and I show that, whilst these earlier ideas are still valid in principle, there are many areas in which present-day stress significantly deviates from plate motion, or the stress expected to arise purely from plate boundary forces and major sources of intra-plate stress. These deviations seem to be either due to different plate boundary forces acting in different directions, mantle drag forces, major structural heterogeneities or additional internal body forces.
Advisor: Tingay, Mark
Heidbach, Oliver
Dissertation Note: Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, Australian School of Petroleum, 2016.
Keywords: present-day stress
plate tectonic
stress regime
plate boundary forces
stress rotation
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