Abstract: Complex Allocthonous Salt Structures and Their Impact on Subsalt Illumination
ALBERTIN, MARTIN L., DAVID SEDGELEY, DAN KIKKERT, JUDY O. MOONEY, ALEXANDER CALVERT, EILEEN MCCARRON, MIKE O'BRIEN, Amoco Production Company, and RICH PRIEM, Priemere GeoTechnology LLC
The allocthonous salt bodies found on the Gulf of Mexico Shelf
(GOM Shelf) act as complex 3D lenses that severely distort wave
propagation. Correct salt and subsalt imaging require 3D depth
migration before stack (3D MBS), but many 3D MBS images contain
areas below salt which are devoid of reliable reflections. Although
inadequate seismic processing
and incorrect velocities are often
cited as causes of poor subsalt imaging, subsalt shadow zones more
often are representative of poor illumination with conventional
surface P-wave seismic.
We use normal incidence ray-tracing, offset 3D ray-tracing, and
finite difference seismic modeling to understand critical angle
effects and acquisition limits common to surveys in regions with
high salt-sediment velocity contrast and complex geologic
structure. The ray-tracing data are captured on illumination maps;
subsalt horizons containing gaps in areas from which no rays can
reach the surface. Using interpretive
workstations, we find that
structural attributes controlling the critical angle (base of salt
dip, and change in subsalt isopach) correlate extremely well with
modeled illumination maps.
We further evaluate the quality of our 3D depth migrations by
comparing the images with migrated finite difference models. If
subsalt shadow zones observed on real data correlate with modeled
illumination gaps, then we gain confidence in the depth image and
ignore questionable events in the shadow zones. Poor correlation
suggests we evaluate other causes, such as processing
errors,
migration parameters, velocity problems, or rock property
effects.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah