Improved Imaging Using Ocean Bottom Seismic in the Snøhvit Field

The StatoilHydro-operated Snøhvit field is currently in the final preparatory stages of gas production. A thin oil column is present below the gas in the area. Poor seismic imaging in the western part of the field, due to shallow gas, has led to uncertainties in both seismic interpretation and hydrocarbon volume calculation. To reduce these uncertainties, ocean bottom cable seismic (OBC) data were acquired in late 2006. The OBC data were processed by CGGVeritas in 2007. A complex 4 component (4C) processing sequence was implemented along with extensive shear-wave statics work and imaging through anisotropic compressional (P reflected to P) and converted (P converted to S) wave pre-stack depth migrations (PSDM). The resulting PSDM OBC data images in the area beneath the shallow gas are significantly better than the images from streamer data. The PP images show that multi azimuth imaging, to a large extent, solves the illumination problems beneath the shallow gas. The PS images do not show signs of the high attenuation effects we see in the PP images. The combination of P and S velocity model building also allowed us to solve the shallow gas in the velocity model.