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