Amplitude variation with azimuth (AVAZ) analysis plays an important role in characterizing both conventional and unconventional reservoirs. The AVAZ technique can provide information about stress and/or existing fractures in the reservoir, thus improving the ability to identify sweet spots and optimize drilling and fracking processes. In the case of fractured reservoir, orthorhombic symmetry can be considered the simplest realistic rock model. The unfractured reservoirs and shale, in particular, can be approximated as a transversely isotropic media with vertical symmetry (VTI). Adding a vertical fracture set to a VTI background rock results in orthorhombic symmetry. Unconventional reservoirs can also display strong reflectivity due to strong contrast between the layers. This is often seen in reservoirs located below high-velocity carbonates. Linearized AVAZ equations assume weak anisotropy and small contrast interfaces. as a results the linearized approximation of the PP-reflectivity will not adequately represent the AVAZ response and nonlinear Zoeppritz equations are required to model the PP-reflectivity correctly. This presentation will focus on the importance and the application of the orthorhombic AVAZ inversion and inversion results from examples from the unconventional play in the Middle East and North America will be shown.


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  1. Gofer, E., R., Bachrach, R., Fletcher, M., Vie
    ., 2016, Nonlinear AVAZ inversion workflow: 86th Annual International Meeting, SEG, Expanded Abstracts, 500–503.
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