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Data acquisition over the Lunskoye gas cloud

Shell International Exploration and Production, Rijswijk, The Netherlands

Corresponding author: Kees.Hornman@shell.com

The first 20% of the full text of this article appears below.

Obtaining a good seismic image under a gas cloud can be a considerable challenge. The gas cloud can cause strong lateral variations in the P-wave velocities, leading to severe ray bending and scattering, and it can cause high absorption and high transmission losses. Lunskoye Field, in shallow waters offshore Sakhalin (a Russian island north of Japan), is one of those fields suffering from imaging problems due to gas above the reservoir, close to the seafloor. As shown in Figure 1, the field is elongated in the N-S direction. A time migration of a 3D streamer survey, acquired in 1996, did show a large wipeout zone over the crest of the structure. This 3D survey was acquired with inlines along the long axis of the field for operational reasons; on the west side the water depths get very shallow, complicating any vessel turns in the E-W direction. In order to correct for the ray bending effects of the gas, a 3D prestack depth migration (PSDM) was carried out in 1997 and these results were a clear improvement over the time migration (Figure 2). However, Figure 3 shows that, even after 3D PSDM, a wipe-out zone with a width of 1.5 km still covers the crest of the structure.

View larger version (54K): [in this window] [in a new window]   Figure 1. Bathymetry map of the survey area. Sakhalin Island is to the west of this map. Lunskoye Field is indicated by dark blue. The rectangular survey outline measures 13 x 25 km.

View larger version (168K): [in this window] [in a new window]   Figure 2. A crossline (E-W) of the time migration with fixed gain (left) and the prestack depth migration (right, displayed in depth) of the 1996 survey.

View larger version (58K): [in this window] [in a new window]   Figure 3. Wipe-out zone after time migration (top) and after prestack depth migration (bottom).