Abstract

Hydrocarbon reserves are increasingly found in areas with complex geology. Accurate imaging of traps associated with pre-salt intervals, thrust belts or inverted grabens is critical at the prospect evaluation stage. These types of zones give rise to strong velocity variations for which the concept of CDP breaks down and, consequently, classic methods of velocity analysis are inaccurate. To reduce exploration risk in complicated geological zones Total has developed an in house 2D pre-stack depth migration and velocity analysis program called MIGPACK. MIGPACK performs finite-difference migration in the source geophone (S-G) frequency domain by alternate downward continuation in depth of geophone and source gathers. Iterative improvement of the velocity model is obtained by an analysis of focusing spots which indicate errors in migration depth.

In this paper a review of S-G migration and velocity estimation by focusing spot analysis is presented. The effectiveness of the method is illustrated with the aid of a modern seismic line from a TOTAL INDONESIE operated block. This line is used to compare performances of conventional post-stack time migration, pre-stack time migration and pre-stack depth migration. The structural image of the prospect is clearly improved by S-G pre-stack depth migration. Better definition of fault planes, identification of pull down and verification of dips are amongst the benefits of this process.