Anisotropy and amplitude versus offset: a case history from the West of Shetlands

Abstract

An amplitude versus offset (AVO) case study is presented from the West of Shetlands, UKCS in which pre-drill AVO signatures over a prospect were interpreted to be expressions of hydrocarbons but upon study of the data provided by the well were interpreted as a manifestation of anisotropy. The core from the well proved to be crucial in understanding the non-hydrocarbon AVO signature.The well encountered good quality, high porosity, water wet sand and an underlying tuff sequence. Neither the dipole sonic log nor the check-shot survey revealed any anomalous elastic properties, nor did they detect the presence of anisotropy. Conventional, isotropic modelling using the measured log properties could not match the observed AVO response. Only comparison of the dipole sonic log and the check-shots with the core measurements gave evidence for the presence of substantial elastic anisotropy. Incorporation of anisotropy inferred from core and log measurements in the shales and tuffs gave a reasonable match to the observed AVO. The inferred anisotropies are substantial, averaging 15% for both the compressional and shear velocities and increasing over some depth intervals to 25%. A subsequent walkaway vertical seismic profile (VSP), acquired for the specific purpose of calibrating the AVO response, shows a marked AVO effect at the base of the target sandstone and tuffs, consistent with the surface seismic data. It is clear from this case study and other work that simple transverse anisotropy, for example, produced by the natural layering in shales, can cause first order effects on the AVO response.