In sand/shale basins explorationists generally correlate by using the traditional technique of comparing the SP and resistivity curve shape on electric logs from well to well. Changes in the borehole environment will alter the shape of the curves, providing possibly erroneous correlations or even non-correlations. Sand bodies are generally the discontinuous phase in a sand/shale sequence, compounding the problem of correlation. It is probable that SP and resistivity curve shape would not be used as a correlation tool if the logging industry had today's technology available at its inception. Well-logging tools quantitatively measure: conductivity/resistivity, interval transit time, natural radioactivity, bulk density, and hydrogen index. These properties in shales may be correlated from well to well, providing proper correlations that are far superior to those afforded by curve-shape comparison. Conductivity/resistivity and interval transit time data are the most available of these measurements (late 1950s to present). In the Puget Trough area of northwestern Washington there is sparse well control and a thick cover of glacial till masking most of the sub-surface geology. Conventional correlation techniques using well logs are ambiguous at best in the am. Plots of linear depth versus the log of conductivity (conductivity-depth plot-CDP) and linear depth versus transit time (transit time-depth plot-TDP) provide definitive correlations that define a large structure. The structural interpretation these plots provide is confirmed by other disciplines, thus illustrating the superiority of conductivity and transit time as correlation tools.

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