Anisotropy of Mudrocks: Quantifying Controls and Fabric Implications in the Horn River Basin

  • Author / Creator
    Ebufegha, Vivian T
  • Shales are widely occurring and highly heterogeneous sedimentary rocks. They exhibit significant variation in composition and distribution of matrix constituents and pores/fracture networks, which can affect their petrophysical and geomechanical properties. These variations are important for determining the integrity of shales as caprocks, and for optimizing hydrocarbon recovery techniques (such as hydraulic fracturing) where shales form unconventional reservoirs. In this thesis, a detailed laboratory study on the application of magnetic and electrical anisotropy in understanding these variations in composition and matrix spatial distribution (petrofabric) in shales and mudstones of the Muskwa, Otter Park and Evie members of the Horn River Basin is presented. Despite increased interest in unconventional shale reservoirs, studies on quantitative anisotropy controls in shales are limited, especially studies comparing results from different quantitative techniques. Three techniques were investigated: (1) Bulk magnetic susceptibility and the anisotropy of magnetic susceptibility (AMS), (2) Electrical resistivity and the anisotropy of electrical conductivity (AEC), and (3) The anisotropy of magnetic remanence (AMR). To test the reliability of these anisotropy techniques as tools for quantitative fabric analysis, their results were compared with one another and with observations from core analysis, thin section analysis, SEM imaging, and geochemical analysis. Bulk susceptibilities of slabbed core for a well penetrating all three members of the Horn River Group showed that the concentration of clay minerals was primarily responsible for the higher magnetic susceptibilities in the Lower Muskwa and Otter Park, while low bulk susceptibilities in the Upper Muskwa and Evie were due to higher total organic carbon (TOC) and quartz concentrations. Whilst the correlations with clay and quartz content was expected, this, to our knowledge, is the first time a correlation between TOC and magnetic susceptibility has been shown. Room temperature anisotropy of magnetic susceptibility (AMS) values determined from 18 directional magnetic susceptibility measurements made on sodium silicate impregnated samples showed good correlation with TOC and gamma ray readings, and little to no correlation with clay concentration. Results indicated that organic matter concentration dictates mineral grain alignment in the Horn River Group. No previous studies have reported a relationship between TOC and AMS. Room temperature AMS principal axes primarily defined normal and inverse fabrics. Inverse AMS fabrics due to uniaxial stable single domain grains were identified for the first time to our knowledge in shales. A comparison with anisotropy of isothermal remanent magnetization (AIRM) principal orientations confirmed the presence of inverse fabrics due to uniaxial stable single domain grains. Previously such inverse fabrics had only been reported in igneous and metamorphic rocks. Low temperature directional magnetic susceptibilities were also measured between -100°C and room temperature and used to determine (i) the illite clay content and (ii) variations in %AMS and principal orientations with temperature. Illite is the main paramagnetic mineral in the Horn River Group, and a series of template curves were developed that allowed the determination of illite concentrations from susceptibility-temperature curves by utilizing the enhancement of paramagnetic susceptibility with decreasing temperature. The illite contents obtained strongly correlated with relevant independent geochemical data from mass spectrometry. Based on the variation of AMS and principal AMS orientations with low temperature, the paramagnetic subfabric was also isolated. When principal orientations remained approximately consistent with temperature paramagnetic minerals had the same orientation as other matrix components. Comparison with anisotropy of magnetic remanence orientations allowed one to distinguish whether inverse AMS fabrics observed at room temperature were due to the presence of stable single domain grains, or ferroan calcite/siderite or a combination of both. Moreover, the low temperature AMS measurements revealed clear and enhanced anisotropy in samples that appeared to be weakly anisotropic at room temperature. This provides a novel method for identifying subtle features like lineations (in shales and other geological samples containing paramagnetic minerals) that may not be recognized from conventional room temperature AMS measurements. The spatial arrangement of pores was quantified in terms of the anisotropy of electrical conductivity (AEC). Principal conductivity orientations suggested that pore spaces are strongly aligned in a bedding parallel direction similar to the mineral components from the AMS principal orientations. The Horn River Group samples were not found to be transversely isotropic with respect to electrical conductivity, and 3D AEC measurements provided a better description of their electrical conductivity anisotropic properties.

  • Subjects / Keywords
  • Graduation date
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
  • Language
  • Institution
    University of Alberta
  • Degree level
  • Department
  • Specialization
    • Geophysics
  • Supervisor / co-supervisor and their department(s)
  • Examining committee members and their departments
    • Schmitt, Douglas (Physics)
    • Dehghanpour, Hassan (Civil and Environmental Engineering)
    • Tony Morris (Geography, Earth and Environmental Sciences, Plymouth University)
    • Kravchinsky, Vadim (Physics)