Stress-induced seismic azimuthal anisotropy offshore NW Australia

Lisa Jade Gavin

    Research output: ThesisDoctoral Thesis

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    Abstract

    Anisotropy is the variation of a physical property at a given point depending on the direction in which the property is measured. Seismic azimuthal anisotropy is observed in many areas of the earth and knowing where it is present is important because it can affect the propagation velocity of seismic waves. Not accounting for velocity anisotropy in processing or inversion of seismic data can lead to incorrect images and physical property estimates, and, therefore, incorrect geologic interpretations. While anisotropy has historically been considered a complication, the effect it has on data can be utilised as a source of information, giving an indication of geological features much smaller than the seismic wavelength. The main research objective of this thesis is to better understand seismic azimuthal anisotropy, especially in the context of the North West Shelf (NWS) of Australia. We explain observations of azimuthal anisotropy across the NWS from the tectonic- to grain-scale and demonstrate the importance of characterizing anisotropy in the context of 3D and 4D AVO modeling and interpretation.

    Earthquake seismology and exploration seismology studies often observe azimuthal anisotropy, as is the case across the NWS. However, we are unaware of any studies that compare fast azimuth polarization directions from exploration-scale to earthquake-scale trends. In this thesis, we first present a regional azimuthal anisotropy analysis using seismic exploration data across the NWS. We determine where azimuthal anisotropy occurs and estimate and map detectable fast polarization azimuths from 34 dipole shear logs and two 3D seismic surveys. We compare the fast polarization azimuth results to in situ maximum horizontal stress directions. Our results show that fast polarization azimuths and maximum horizontal stress direction trends correlate across a geographical area spanning almost 2,000 km, which compares well with published results in the region from earthquake seismology studies. Our results suggest that differential horizontal stress is the likely physical mechanism creating azimuthal anisotropy at a wide range of spatial scales across the NWS. We also show that azimuthal anisotropy is not observed in exploration data in some areas of the NWS, and propose a lithology-dependent stress-sensitive anisotropy model, in which azimuthal anisotropy is strongest in clean, unconsolidated quartz-dominated sediments, and weak or non-existent in deformable shales, and cemented rocks such cemented sandstones and carbonates.

    The majority of literature focuses on azimuthal anisotropy resulting from vertically aligned fractures. There are currently no physical models available that we are aware of to describe how azimuthal anisotropy induced by differential horizontal stress varies with sand-shale content or depth; we develop a model that reproduces S-wave azimuthal anisotropy observations in unconsolidated sand-shale sequences offshore NW Australia. Our method naturally introduces two new concepts: critical anisotropy and anisotropic depth limit. Critical anisotropy is the maximum amount of azimuthal anisotropy expected to be observed at the shallowest sediment burial depth, where the confining pressure and sediment compaction are minimal. The anisotropic depth limit is the maximum depth where stress-induced azimuthal anisotropy is expected to be observable, where the increasing effects of confining pressure and compaction make the sediments insensitive to differential horizontal stress. We test our model on borehole log data acquired in the Stybarrow Field, located in the NWS. We derive our model parameters in one well and use them to accurately predict the azimuthal anisotropy values at two other wells in the same area. This accurate prediction demonstrates that our model is useful for predicting S-wave anisotropy in wells where there is no dipole shear log data available. We demonstrate the importance of our prediction model to improve the match of angle-dependent reflectivity amplitudes to the 3D seismic data at the well location. A future implication of this work is that sand quality could be estimated from anisotropic parameters derived from 3D seismic data.

    Finally, we consider the effects that azimuthal anisotropy may have on 3D and 4D amplitude versus offset (AVO) modeling, analysis and interpretation. This is especially important in the context of the NWS; as we show azimuthal anisotropy is extremely prevalent across the region and likely to occur in clean sandstones (for example, significant petroleum and CO2 storage reservoirs). We outline the theory to model the amplitude-versus-offset and azimuth (AVOA) response for the combined effect of fluid changes within a reservoir and a difference in acquisition azimuth, in the presence of azimuthal anisotropy. We then show that the observed 3D and 4D AVO data from the Stybarrow Field cannot be fully explained without incorporating azimuthal anisotropy. This work demonstrates the importance of accounting for azimuthal anisotropy in 3D and 4D seismic modelling, inversion and interpretation studies.

    In summary, the research presented in this thesis demonstrates that seismic azimuthal anisotropy is prevalent across the North West Shelf, Australia, in unconsolidated sandstones and most likely induced by differential horizontal tectonic stress. The analyses, methods and techniques we develop are also likely to be useful in other regions.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    • The University of Western Australia
    Supervisors/Advisors
    • Lumley, David, Supervisor
    • Shragge, Jeffrey, Supervisor
    Award date1 Jul 2016
    Publication statusUnpublished - 2015

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