A Multi-Offset Vertical Seismic Profiling (VSP) Experiment for Anisotropy Analysis and Imaging

Abstract

Editors Note Editor's note: The Canadian International Petroleum Conference held June 4 - 8, 2000, featured a Graduate Student Presentation Competition as part of the Petroleum Society's tradition at the Annual Technical Meeting. Graduate students from respected universities across North America competed in these sessions. The objective of the competition was to give participants the opportunity to present highlights of their work to petroleum industry experts from around the world. The presentations were judged on technical content, applicability, and overall presentation quality. Judges included Dr. Nick Mungan, Mungan Petroleum Consultants Ltd.; Dr. Don Towson, IRAP; and Dr. Norman Freitag, Saskatchewan Research Council. A "Best Graduate Student Presentation Award" and two runner-up awards were presented by Don Mallory, chair of the Student Competition, shown here with the 1st Place presentation author, Graziella Grech of the University of Calgary. On behalf of the Petroleum Society, the JCPT is pleased to present Ms. Grech's award winning presentation. Abstract A multi-offset Vertical Seismic Profiling (VSP) experiment was carried out in the Rocky Mountain Foothills of Southern Alberta, Canada. The purpose of this survey was to determine whether the dipping shale strata in the study area exhibit seismic velocity anisotropy, and to obtain an estimate of the error in the imaged location of the target underneath these shales if anisotropy is not taken into account during seismic data processing. Traveltime inversion of the first arrival data from the multioffset VSP has revealed that the shales exhibit velocity anisotropy of about 10%. For a target depth of about 3,000 m and moderate dips of 30 ° to 50 ° in the anisotropic overburden, this may lead to a lateral shift in the imaged location of the target of up to 300 m in the up-dip direction of overlying bedding. The anisotropic parameters derived from this study will also be used in the anisotropic prestack depth migration of the VSP and surface seismic data. FIGURE 1: Interbedded thin layers of sandstone and shale found in the Western Canada Sedimentary Basin. Due to anisotropy, the velocity parallel to bedding (V90) is higher than the velocity perpendicular to bedding (V0). This class of anisotropy is termed Transverse Isotropy. (Photo courtesy D. Spratt) (Available in full paper) Introduction Vertical Seismic Profiling (VSP) and surface seismic data are used to image and locate hydrocarbon targets in the subsurface. It is well established that these depth images depend on the accuracy of the velocity model(1,2). If rocks overlying the target are anisotropic, and if this property is not accounted for during velocity model building and depth imaging, then the final image will be incorrect, hence increasing the risk of dry holes. It is therefore important to determine which formations exhibit seismic velocity anisotropy and quantify their parameters for use during seismic imaging. Seismic Velocity Anisotropy Seismic velocity anisotropy is the change in velocity with the direction in which it is measured. In rocks, it can be caused by the alignment of mineral grains, cracks and crystals in a preferred direction, and by the consecutive layering of thin beds.