Velocity-Depth Model Estimation for a Subsalt Target from the Southern Gas Basin of the North Sea

Oz Yilmaz, Jeff Rutledge, Ottar Sandvin, and Bob Godfrey

The Southern Gas Basin of the North Sea has been subjected to extensional tectonics, primarily in the east-west direction. Subsequent occurrence of the salt diapirism gave rise to the presence of complex structures. By doing a depth-domain analysis of a 3-D seismic survey data from an area in the Southern Gas Basin, we delineated the structural geometry of the top Rotliegendes formation beneath the complex Zechstein diapiric formation. This required an accurate estimate of the velocity-depth model above the Zechstein diapiric formation and removal of its deleterious effect on the underlying Permian sands of Rotliegendes and deeper targets. We conducted a layer-by-layer depth. domain analysis, and used coherency inversion to estimate layer velocities and 3-D poststack depth migration to delineate reflector geometries down to top Zechstein. We ver fied the accuracy of the velocity-depth model for the overburden above Zechstein by analyzing image gathers from prestack depth migration. We then analyzed constant-half-space image-gather stacks to estimate velocities for the substratum including Zechstein and the underlying Carboniferous sequence. Finally, we performed 3-D prestack depth migration to delineate the base Zechstein-top Rotliegendes geometry.

This study demonstrates the need for depth-domain analysis of seismic data to derive accurate structure maps for targets beneath complex structures associated with salt and overthrust tectonics. The final output from depth-domain analysis -- a velocity-depth model, can then be used as a canvas for a reservoir model.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California