Previous log examples demonstrate that two logging measurements are reputed to respond to the difference between clean and shaly formations. One of the measurements, the SP (see Chapter 3), has been analyzed in some detail, and it is known to have a marked response to clean permeable zones. This is evident in the right track of Fig. 11.1 where the more negative potential of the zone between 8,510 and 8,540 ft is due to the simultaneous absence of clay and free communication between the borehole fluid and the formation waters. For the same well, on the left side of the figure, the gamma ray log (or GR) has a structure similar to the SP trace: a low reading in the clean zone and a high reading in an apparently shaly zone. Figure 2.18 shows another example, in track 1, where the GR log and SP correlate to a high degree.
KeywordsClay Zircon Attenuation Sandstone Radioactive Isotope
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