Application of Image Ray Tracing in the Southern North Sea Gas Fields

Abstract

Summary The application of Time Migration to seismic data has won a great deal of popularity due to its ability to better locate seismic reflection events. Commercially available Time Migration algorithms all involve different approximations to the Wave Equation. In practise these simplifying assumptions work effectively in areas such as the Northern North Sea, where the shallowest reflection with a significant structural dip is often close to the target horizon. In the Southern North Sea Gas Basin, however, the section which overlies the Top Rotliegendes target horizon commonly shows a large degree of structural dip. Due to the resulting high lateral velocity variation the wave equation approximations of Time Migration are no longer valid, and this results in the incomplete imaging of the seismic data. Depth Migration potentially offers a complete solution to full imaging of seismic data beneath a section with laterally varying velocities. However, in either its pre-stack or post-stack mode, Depth Migration is an expensive iterative process. In certain circumstances incomplete imaging of the Time Migrated data can be accommodated by application of Image Ray Tracing techniques. The repositioning of subsurface structures demonstrated by both techniques could potentially influence reserves, development plans, and equity determinations. This paper contains Image Ray Tracing examples from the Southern North Sea Gas Area. It discusses a practical procedure to incorporate the modelling results, and highlights situations where this technique has proven to be successful, and others where problems with interpreting the results of modelling still exist.