Enhanced Seismic Interpretation Guided by Multi-Azimuthal Borehole Seismic Data

Abstract

In the oil & gas industry, surface seismic imaging is critical in defining the sub-surface structure & stratigraphy but could suffer from poor seismic resolution due to complex geology. This paper describes applications of Walkaway and Walkaround VSPs recorded over an oilfield with low surface seismic resolution located in the offshore Arabian Gulf. Two further challenges needed to be addressed: i) estimation of azimuthal anisotropy, and ii) calibration of PS seismic data. Walkaway VSP was proposed as a viable solution to obtain the higher seismic resolution by placing the receiver below the strong shallow reflectors. A multi-azimuthal (4 orthogonal lines) walkaway VSP survey was acquired based on pre-survey modeling to achieve the objectives of higher resolution and calibration of PS data obtained during OBC acquisition. Walkaround VSP was also acquired in two settings: a deep setting to estimate the azimuthal anisotropy to provide information on presence of natural fractures in the main producing reservoirs, and a shallow setting to investigate the causes of a major lost circulation zone. 3-Component downhole receivers and a powerful seismic source (6-Gun Cluster) were used to acquire good quality signal below the near surface attenuating layers. Passing through an elaborate 3-component processing chain, the walkaway survey resulted in hi-resolution subsurface PP and PS images (both in time & depth). These images provide detailed sub-surface features which were not observed in the existing surface seismic, resulting in improved reservoir characterization, as well as calibration of the surface PS seismic data. Walkaround VSP data were interpreted in terms of fracture azimuth (Fast Shear direction) and fracture density variation with depth (ratio of transverse to radial amplitudes). The Walkaround VSP results show good correlation with core data across two discrete reservoir intervals. Walkaround VSP data recorded across the shallow lost circulation zone showed strong anisotropy that coincides precisely with the lost circulation zone indicating high fracture density, with the fast velocity azimuth being consistent with the regional stress field. It was the first time that high resolution VSP images (vertically and spatially) were generated for this field. High resolution walkaway VSP images were utilized to enhance the existing horizon and fault interpretation and reservoir characterization. The Walkaround VSP also helped to understand the azimuthal anisotropy and presence of natural fractures near the well in the main producing reservoir.