In early 1998, Chevron acquired one of the world's first full-field four-component ocean bottom cable (3D/4C OBC) surveys at the Alba field in the central UK North Sea. The primary objective of the survey was to use converted shear waves to provide a better image of the sandstone reservoir and shales within the reservoir. Pre-survey technical studies based on a dipole sonic log and 2D OBC seismic lines gave us confidence that converted waves (PS) could provide a better image of the reservoir relative to conventional P-wave seismic data.
The secondary objective of the new survey was to map water movement in the reservoir after four years of production and water injection by comparing the new P-wave OBC data with the original 1989 streamer data. A strong original oil-water contact reflector seen throughout much of the field and pre-survey technical studies suggested that production-related saturation changes would be observable on the new P-wave OBC seismic data.
The new data shows that both objectives were achieved with dramatic results - the converted-wave images provide the clearest image of the Alba reservoir sands ever seen and production effects are obvious on the new P-wave OBC data near several producing and injector wells. In addition, the converted-wave data has offered new insights into the shape of the reservoir. The previous lens-shaped interpretation based on P-wave data has been replaced with a far more complex shape that is at least in part related to post-depositional structural alteration. This new interpretation is also supported by improved reservoir images seen on far-offset P-wave sections.
To date, two successful wells have been drilled based primarily on the converted-wave seismic data. Both wells prove the existence of ‘wings’ – structurally high sand at the margins of the channel that may represent re-mobilized and injected reservoir sand. Neither of these wings had been previously identified on the conventional P-wave streamer seismic data.