Large-scale Application of (Anisotropic) Pre-stack Depth Migration in the Foothills

Abstract

Abstract Introduction Shell Canada Limited is routinely applying full (anisotropic) Pre-Stack Depth Migration (PSDM) to 2D and 3D seismic surveys in the Foothills of Western Canada. In this paper we will show data examples which illustrate the practical application of these techniques. This technology can contribute to drilling success though proper well positioning and thus has important business implications. Anisotropic 3D PSDM In the highly structured fold and thrust belt of the Foothills, anisotropic clastic beds which are steeply dipping often overlay deeper carbonate targets. This can result in poor focussing and mispositioning of structures on the seismic section unless these effects are correctly accounted for in processing. The Shell Group has developed algorithms for anisotropic prestack depth migration which are being effectively applied by Shell Canada on both 2D and 3D Foothills data. Structures show improved focussing and, in some cases, lateral shifts on the order of 100 - 300 metres. The formulation of the Shell proprietary anisotropic PSDM is such that it is able to handle any symmetry of anisotropy. However, current experience is limited to working with the TI assumption, though also accommodating a laterally varying tilted axis of symmetry. Practical considerations for PSDM Shell's proprietary state-of-the-art processing management software (used worldwide by the Shell Group) deals with many of the logistical requirements for processing large amounts of data on a network of workstations. This has had significant impact on the turnaround achievable for 3D PSDM projects. It also frees up geophysicists to concentrate on geophysical issues, including velocity model building and updating, as opposed to administering jobs through the system. Thus quality PSDM processing is accomplished in a timely, efficient manner which is suitable not only for Development but also for Exploration needs. Though expertise is required to oversee the correct application of the technology, the whole system facilitates a staged progression of learning for personnel new to 3D PSDM, allowing them to quickly become productive. Our PSDM processing software is tightly linked into Shell's proprietary 3D interpretation software (123DI) which provides a communication vehicle for processor/interpreter interaction on an ongoing basis from initial model development to intermediate results and model updates, to final product. This is a crucial link, the impact of which should not be underestimated.