Spatial variation in azimuthal anisotropy due to sand-shale distribution: Stybarrow Field case study

Abstract

Anomalous horizontal stress conditions within reservoir rock can result in azimuthally anisotropic elastic properties, the effects of which can be observed in borehole and 3D seismic data. The amount of stress-induced anisotropy can vary depending on sedimentary rock type and on subsurface stress conditions. There is currently no methodology we are aware of to quantify the relationship between azimuthal anisotropy and sand-shale content in unconsolidated sediments. We use data from the Stybarrow Field, located offshore NW Australia in the Carnarvon Sedimentary Basin, which is an area where strong anomalous horizontal stress conditions are present and have induced azimuthal anisotropy. We model azimuthally anisotropic gathers from logs and spatially correlate azimuthal AVO variations with sand-shale content. We derive a relationship that predicts the Thomsen parameter gamma as a function of the shale volume (for volume ratios greater than 0.3), and use this relationship to predict spatial variations in the azimuthal anisotropic AVO observed in the seismic data at Stybarrow. Implications from this work are that azimuthal anisotropy can be a strong function of sand-shale content, and that spatial variations in the sand-shale ratio can be estimated from azimuthal AVO.