Imaging challenges

Several imaging challenges associated with Alwyn are well illustrated in Figure  that shows the seismic crossline of the migrated cube obtained by CGG in 2003 after isotropic Kirchoff prestack migration. It shows the following different challenges encountered in the area of interest:

• First, the reflectors and faults below the BCU horizon (strong reflector at around 3.5 km depth) are moderately well focused.
• Second, a complex channel at around 1.0 km depth distorts the underlying horizons and especially the Balder horizon (strong and continuous reflector at around 1.5 km depth), affected by pull-ups (for example, at inlines 630, 660 and 700).
• Third, strong multiples contaminate the lower part of the image as well as strong migration smiles due to limited crossline acquisition aperture.

Furthermore, anisotropy is commonly observed in the North Sea. In the Alwyn field, the thick layer between the Top Chalk horizon (at around 2.2 km depth) and the BCU horizon is made of chalk, a sedimentary carbonate rock whose main mineral (calcite) has an anisotropic crystalline structure. The anisotropy of this layer, if not taken into account in the migration, can result in poor imaging of the underlying layers.

 

Section_Crossline_1999
Figure 8 Crossline 1999 of CGG 2003 PSDM cube obtained after isotropic Kirchhoff prestack migration. The distance between two inlines is equal to 18.75m.