3-D Salt and Sub-salt Imaging Strategy: A Case History from the Gulf of Mexico

The purpose of this paper is to demonstrate 3-D prestack depth migration as an ultimate technique for imaging structural targets associated with salt diapirism, and compare it with other migration strategies — 2-D prestack, and 2-D/3-D poststack depth migrations. The structural problem is to image sedimentary bed terminations, bottom salt boundaries and undersalt layers around two large salt masses in the Gulf of Mexico. A large number of oil and gas reservoirs in the Gulf of Mexico is controlled by salt structures. Typical salt flank plays have been mapped quite successfully with time migration strategy. However, as exploration moves into more complicated targets around salt overhangs and subsalt traps, depth migration strategy becomes necessary.

The first task is to build a detailed 3-D velocity-depth model. To accomplish this, the following robust approach has been chosen. Using a velocity-depth model for the overburden (sedimentary section) based on edited stacking velocities, 3-D poststack depth migration was performed to verify its accuracy. Iterative 3-D poststack depth migration then followed to define the salt masses. Top salt reflectors were interpreted in 3-D and constant salt velocity was assigned to the half-space below the top-salt. Again, 3-D poststack depth migration was performed and base-salt events were interpreted. Finally, the velocity model was updated to contain complete salt bodies and 3-D poststack migration was rerun to check results. This loop of three iterations was repeated until convergence was achieved — the input model matched the result of the depth migration.

Given the velocity model, a single output line was selected and 3-D dynamic raytracing was performed to link output image points with surface shot-geophone locations. Finally, 3-D Kirchhoff depth migration was run in 3-D prestack, 2-D prestack and 2-D poststack modes to produce comparison sections along the output line.