Vertical seismic profiling (VSP) is a key tool for obtaining time-depth information in wells. Over the past couple of decades VSP has also been used to image areas where surface seismic imaging fails. In this paper, walk-away VSP data from a prospect in the deep water Gulf of Mexico are presented. The data comprise 40 geophone levels and about 600 shot points within +/- 30,000 ft offset.
The processing of the VSP data yielded a high-resolution image of base salt as well as several layers where the well and base salt intersects. These details cannot be seen in the surface seismic images. Interpretation of these data suggests the presence of salt imbrications and entrainment of sediments. During the drilling through this section, problems such as overpressure and mud circulation losses eventually led to the well being abandoned.
The VSP processing also yielded a detailed image of the dipping subsalt layers, which are not locally visible on the existing surface seismic images. By undershooting the salt overhang we were able to illuminate the target area with superior quality and provide confirmation of the geologic dip interpretation. The imaging results were of such high quality that the partnership was motivated to redrill the prospect. The redrill location was chosen so as to avoid the complexities revealed by the base salt VSP imaging.
The subsalt imaging was performed with geophones located inside the salt body, as opposed to previously published data on subsalt VSP imaging1, where the geophones were positioned below base salt. These VSP data demonstrate that high-resolution sub salt imaging can be successfully obtained with the geophones positioned inside a salt body.
The original plan for the well was to drill through a salt overhang into layers at depths between 24,000 and 30,000 ft below sea level. Unfortunately, the well was abandoned after several attempts to drill through the base salt. The VSP survey was designed to image both the base salt and the dipping beds of the target area, the subsalt. The results of the VSP imaging provide a better understanding of the problems encountered when drilling the well.
VSP is traditionally used to obtain time-depth information and to image the immediate vicinity of the well bore (corridor stack from a zero offset geometry). By moving the source away from the well we can illuminate a larger area around the well. For many prospects, obstructions such as salt bodies or gas clouds create imaging problems2,3. VSP can illuminate targets with superior quality compared to surface seismic by undershooting such obstructions. In many cases the VSP data can be obtained and processed prior to drilling through these obstructions. This would help determine both where to best set the casing and where to drill.
It is important to perform both 3-D and 2-D survey modeling prior to any data acquisition in order to optimize the source and receiver locations4.