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ABSTRACT: Planning and Interpretation of a VSP in a Strawn Mound Prospect, Lea County, New Mexico

Cynthia E. B. Sullivan

A zero offset and single-source offset vertical seismic profile (VSP) was conducted in an 11,500-ft Strawn mound test in Lea County, New Mexico. The VSP was done to achieve (1) an accurate well tie, (2) a representation of the mound away from the borehole, and (3) a comparison with a key surface seismic line.

Good planning of the VSP was critical to accomplish these goals and, invariably, the interpretation process exposes any weakness in the planning process. The planning phase included forward modeling in SIERRA to test whether the VSP could detect changes in the mound configuration and porosity content. Survey parameters were designed to approximate surface seismic in order to acquire data with a close match. The offset was run parallel to the key surface line so that a comparison could be made. However, we did not reprocess the key line using a deconvolution operator designed from the VSP and therein lay the main problems encountered in the initial interpretation process.

The zero offset was most valuable as a well tie in this difficult seismic stratigraphic play. The single source offset was mapped and binned to approximate a mini-seismic section, and it compared well with the surface line. The main problem was a difference in phase: the surface line was out of phase with respect to the VSP which was zero phase. Because the target zone is contained within one-half to one cycle of data, and a prospect is determined by subtle amplitude changes within that cycle, any differences in phase must be resolved. Two-dimensional forward modeling using a Tektronix was employed to examine seismic response from the two different data types: VSP zero phase and surface line mixed phase. This method helped resolve interpretation problems. The offset VSP helped identif a possible carbonate mound north of the borehole. Processing of available seismic with a deconvolution operator designed from the VSP data was recommended.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990