Time and Depth Migrations Over Giant Lunskoye Gas Field, Offshore Sakhalin Island, Russian Federation

Sakhalin Energy Investment Company Ltd. (SEIC) acquired a new 3D seismic survey over the giant Lunskoye gas field in 2003, orthogonal to the 1997 N-S 3D survey. The survey was acquired within time and budget, with an extensive environmental program to ensure minimal impact. The new survey attempted to undershoot the gas cloud, which extends N-S along the Lunskoye crestal axis. A high-quality, Shell-processed, prestack time migration (PSTM) volume was produced and interpreted followed by two prestack depth migration (PSDM) volumes. The first PSDM, in 2005, extended to just below the penetrated reservoir depth interval and used a velocity field without an interpreted low-velocity gas ‘body’. The second PSDM, in 2006, was extended in depth and migrated with an interpreted shallow gas body in the velocity model. The structure and structural uncertainties derived from the 2005 PSDM were used in the SEIC's Integrated Reservoir Model (IRM) project. Structural uncertainty was constrained by data from all seven wells and through the integration of seismic amplitude information and fluid contacts derived from petrophysical data. Three discrete structural cases - low, reference and high – were interpreted which honored these constraints. A number of comparisons were made between the 1997 and 2005- 2006 orthogonal depth migrations. Quality of seismic reflectivity over the crest improved dramatically. Faulting outside the gas cloud area has remained fairly consistent between the datasets. Time-to-depth conversion will be discussed and differences in migration velocity models also presented. Reliable reservoir models depend, to a large degree, on the quality of seismic data used in seismic inversion. In an area where a severe gas cloud obscures proper reservoir imaging, all improvements to the structural and stratigraphic image will have a positive impact on the reservoir model.