Application of Kriging to the Mapping of a Reef from Wireline Logs and Seismic Data; A Case History

Abstract

Maps constructed from seismic data represent subsurface structures in terms of two-way transit time. Wireline logs provide depth of formation tops at well locations. The best representation of a subsurface horizon should combine both measurements. Kriging is an ideal tool for merging seismic and log data.

This case history describes the mapping of a reef in north Texas. Seismic data include three common depth point (CDP) lines and a large number of single-fold records. Wireline logs, including a vertical seismic profile (VSP), are available in 11 wells.

The first step is the geostatistical analysis of seismic data. The empirical semivariogram is drawn from several hundred shot points, and a theoretical model is fitted. This semivariogram is used to construct a time grid.

In the second step, wireline logs are correlated to the seismic sections with the help of the VSP data. A time-depth relationship is established and applied to the time grid to yield a “drift” for the kriging of the depth data. The final output is a depth grid used to generate a contour map.