Besheli, Said Amiri (Department of Exploration Geophysics, Curtin University of Technology, Australia) | Urosevic, Milovan (Department of Exploration Geophysics, Curtin University of Technology, Australia) | Li, Ruiping (Department of Exploration Geophysics, Curtin University of Technology, Australia)
The potential of using Amplitude Versus Offset-and- Azimuth analysis (AVOAz) for characterizing seismic anisotropy of a clean sand reservoir is shown to be critically dependent on an accurate assessment of the anisotropy of the sealing rock formation. An example from Exmouth Sub-basin, offshore North-West Shelf, Western Australia is used here to show that seal anisotropy can have the first order effect on AVO gradient for moderate to far offset range. Moreover, significant variation of the seal anisotropy across the field further complicates AVOAz response and makes characterization of the reservoir properties and fluid distribution difficult tasks. To alleviate this problem the anisotropy of the overlaying seal unit was initially estimated from matching the field data with anisotropic AVO modeled responses in the direction of the principal horizontal stress (sH). In this direction we expect that the potential effect of reservoir sand azimuthal anisotropy is minimal (plane of symmetry). Thus, before performing detailed AVOAz analysis to assess reservoir properties , relative variations of seal (VTI) anisotropy across the entire field was performed. Thomsen anisotropy parameters ε and δ were found to vary between 0.15-0.3 and (-0.1)-0.1, respectively. This relative variation of seal anisotropy across the field was finally calibrated at the wells location by utilizing a joint inversion approach.