Gary W. Coburn1, Jeffery M Schneider2
(1) Bell Geospace, Inc, Houston, TX
(2) Texaco, Bellaire, TX
ABSTRACT: Imaging Salt and Sub-Salt Sand Fairways in the Deepwater Gulf of Mexico using 3D Full Tensor Gradient Enhanced Seismic Interpretation
The Green Canyon / Walker Ridge area contains numerous salt bodies. The salt movement and withdrawal has formed very large subsalt structures. Interpretation of the base of salt and the subsalt structures is extremely difficult, due to the poor quality of the subsalt imaging of seismic data. Factors affecting subsalt imaging are: Steep dips of the top of salt, rugose top of salt, salt structures with embodied sediments, multiples and velocity insensitivity.
To define the salt bodies both vertically and laterally as well as the subsalt structures, full tensor gradient data was collected. Texaco's interpretations of the top of salt and base of salt were converted to depth and a 3-d model constructed. Gradients were calculated and compared to the measured data via a differencing technique. Density grids, as well as the extent of the top of salt and the depth of the base of salt were modified based on the difference maps. The process was repeated until a difference of 3 Eötvös +/- was achieved. New Top of Salt, Base of Salt and Salt Isopach Maps were then generated.
The FTG analysis showed two deep-seated "keels" in the southern salt body and a rooted salt to the north. The FTG data detected a subsalt sand fairway that was subsequently confirmed by seismic data. This sand fairway masked the negative anomaly, which should have been present due to the salt, by creating a positive anomaly, due to the higher densities of the subsalt sands. Numerous overhangs are present in conjunction with the deep-seated salt bodies. Subsequent seismic processing done with the benefit of this study, has resulted in much improved base of salt and subsalt imaging which fully supports the conclusions reached by the FTG analysis.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado