18th Oil Shale Symposium, Grand Junction, Colorado, April 1985
Authors: H. A. Perry (Exxon Company, U.S.A.), and C. E. Brechtel and M. P. Hardy (AAI)

Fourteen close-spaced core holes were drilled through the Mahogany mine zone on the Colony property to evaluate lateral oil shale grade variability. Lateral grade variation was evaluated using horizontal variograms, developed for the average grade of the 60-ft Mahogany mine zone, the 25-ft upper heading, and 35-ft lower bench. Variograms for the close-spaced holes did not indicate any reduction from the overall sample variance at the minimum spacing of 90 ft. However, the variance of the close-spaced holes as a group is roughly one-half the overall variance of all 59 holes located throughout the property for the 60-ft zone and the lower bench. The variance of the close-spaced holes is similar to the overall variance of all holes for the upper heading. The variograms indicated that predicting retort-feed-grade variability with vertical boreholes drilled from the surface would be impractical. The evaluation of the grade of daily or monthly production would require the development of some type of in-mine sampling program. The shape of the variograms for all the drilling data was not consistent with the variogram models commonly employed in geostatistics. This was probably due to the relatively small number of holes that are available for the analysis, especially at close and intermediate spacings. Thickening and thinning of stratigraphic subzones in the Mahogany mine zone was investigated in an attempt to identify the mechanism for lateral grade variability. Correlations between average grade of the subzones and thickness were poor, indicating that this was not a primary reason for lateral grade variation.

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