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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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This paper is part of an ongoing study to assess the performance of a natural sandstone reservoir considered for underground storage of gas. The specific problem dealt with is contour mapping the top of the structure in order to assess the closure zone. Data are from 56 wells, most of which were drilled in the central part of the dome, and four seismic surveys. A particular difficulty encountered is the small number of reliable velocity measurements.
The method used is "kriging," an optimal interpolation procedure based on random-fields theory. Its advantage over conventional methods is the use of the variogram, a structure function that depicts the spatial variability of the phenomenon under study. This method results in an interpolation algorithm tailored to each particular data set. Furthermore, error variances are attached to kriging estimates, telling how reliable the estimates are.
Depths to the top of the structure are obtained by adding estimated thicknesses of the intermediate layers to estimated depths of the seismic marker. Kriging of the thicknesses is performed after aggregation of the intermediate layers into approximately uncorrelated sets. For seismic-marker depths, three different methods, combining seismic and borehole information, can be used.
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