Multifarious VSP methods for imaging of complex structures with three component data

Yingping Li, Baker Atlas, Dorothea Faw, Ocean Energy, James Jackson, David Dushman, Fran Doherty, Baker Atlas provide a case study of the benefits of modern borehole seismic technology in the Gulf of Mexico In the US Gulf of Mexico, petroleum reserves are often trapped underneath overhangs of diapiric salt domes. Although the top of salt and sediment structures can be well imaged using current surface seismic migration techniques, the interpretation of the salt flank with irregularly shaped overhangs and the position of updip sands is often ambiguous. Therefore, vertical seismic pro-filing (VSP) surveys are usually requested for additional, independent information (Sparlin and Deri, 1989; Millahn and Manzur, 1990; Nosal et al, 1991; Barker and Guzman, 1992; Burch and Johnson, 1993). The area under study is located in Eugene Island Block 126. There was a deviated well drilled to 11 794 ft SS in the vicinity of a salt dome. A comprehensive borehole seismic survey was designed and acquired in this well in order to obtain a better image of the updip sands and the salt flank. The multifarious three component (3C) VSP data consisted of a rig-source velocity survey (VS), an offset VSP (OVSP), and a refraction salt proximity survey (SP). A map view of the VSP survey configuration is shown in Figure 3. The blue circles are source locations and the pink triangles represent the well trajectory. The survey source was a 500 in, 3 fourgun tuned airgun array. A DGPS navigation system was used to ensure that all sources were in the correct position. To reliably estimate arrival directions for the SP, a gyro tool was attached to the 3C geophone and was used to monitor the orientation of the horizontal geophone components. The rig-source VS was acquired to accurately measure the sediment velocities needed for processing the OVSP and SP data. The data were also subsequently used for ray-trace modelling. The 3C OVSP was shot at an offset of 5571 ft from the wellhead. Presurvey modelling indicated the optimal position of the source to produce reflection images for both the salt face and the updip reservoir sands flanking the salt. The source location of the SP was positioned over the salt top at an offset of 5404 ft from the wellhead. The SP source was positioned in order to utilize the refracted waves to define a salt face position by calculating salt exit points in 3D space. In this paper, we present VSP imaging results from the multifarious VSP survey in the Eugene Island field. The OVSP reflection image of the salt face closely matches the salt exit points calculated from the 3D SP. The determined salt boundary is also consistent with existing well control data. The sedimentary reflectors from the OVSP data terminate as they approach the salt face. Post-survey travel time modelling of the SP data and the salt lead from the VS data provided a further constraint on the shape of the salt flank. Both the refraction and reflection images can be integrated with other available information such as well control data, results from ray-trace modelling, and surface-seismic data, to produce a consistent image of the salt face and the target sand beds. Careful processing and interpretation of the various VSP data result in the precise definition of a diapiric salt dome and updip sands in the Eugene Island field.