Relationship between shear wave azimuthal anisotropy, sand-shale content and depth in the Exmouth Sub-Basin, Western Australia

Abstract

Anisotropy characterisation from surface seismic, VSP and borehole measurements is notoriously difficult. This is mostly due to limitations in parameterising the stiffness tensor constants required give an accurate representation of the anisotropic material. The Exmouth Sub-Basin has very strong anomalous horizontal stress conditions; in some areas the maximum horizontal stress (σhmax) can exceed the vertical stress (σv). There are many data examples across the Sub-Basin showing the influence of strong azimuthal anisotropy (AA). Qualitative observations of log data demonstrate that there is greater amounts s-wave anisotropy (ϒ) in the sand dominated sediments than in the shale dominated and the AA is greater in shallower clastic sediments than deeper. In this paper we compare data observations of AA fast and σhmax azimuths in the Exmouth Sub-Basin, which show consistent trends when plotted geographically. We then outline a methodology to determine a relationship between ϒ and Vshale (shale volume ratio versus sand and shale volume) from logs with existing ϒ and Vshale data and then extend the method to incorporate the total vertical depth (subsea) of the log (TVDss). We test this methodology on a well taking a "blind test" approach, predicting Vsslow, ϒ and Vshale. We also test the methodology on original logs and ones upscaled using Backus averaging to a typical seismic resolution. Our results show that we can accurately predict AA and Vshale, especially when the logs depth is incorporated, removing depth trends.