Seismic azimuthal anisotropy analysis for estimating reservoir properties at Stybarrow Field, NW Shelf, Australia

Abstract

Reservoir rocks are often subjected to anomalous vertical and/or horizontal stress conditions and may also contain complex physical attributes such as fracture sets. These rock properties are not easy to detect and map directly away from the borehole, but are sometimes indirectly evident in seismic data as azimuthal anisotropy. Thus, an analysis of anisotropic rock physics seismic attributes can be important for estimation of stress orientations and magnitudes, useful for reservoir evaluation, reducing drilling risks, and enhancing reservoir injection and production methods. The Stybarrow field, located offshore NW Australia, is an example where seismic data exhibit strong azimuthal anisotropy, likely due to the effects of large horizontal tectonic stresses applied to the Carnarvon sedimentary basin. We find evidence for azimuthal anisotropy in AVO reflection amplitude difference maps and cross plots from two repeated 3D seismic surveys acquired at different azimuths, as well as in dipole shear logs and borehole breakout data. We model azimuthal AVO responses using Ruger?s horizontal transverse isotropy (HTI) AVO equation, using the anisotropy parameters derived from dipole shear logs, and compare the results with AVO gathers from the two 3D seismic surveys with different acquisition azimuths. We use a least squares method to find the coordinate rotation and scaling factor that optimally matches the real seismic data to the modelled data predicted by Rüger?s theory.