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Dakota Aquifer Program--Petrophysics

Geophysical Log Analysis of the Dakota Aquifer

The Gamma-ray Log

The gamma-ray log is widely used as a record to locate the depth of key stratigraphic formations (Figure 2) and to subdivide the Dakota into units of sandstone and shale (Figure 3). The gamma-ray tool measures natural radioactivity of rocks in a similar way to a geiger counter. The sources of radiation are almost entirely from isotopes of thorium, uranium, and potassium. Although the radioactivity of most rocks is fairly low, it is sufficient to make a clear distinction between sandstones (low radioactivity) and shales (higher radioactivity) (Figure 3). Older gamma-ray logs are recorded in "counts" whose numbers vary according to the tool design. Almost all modern gamma-ray logs are recorded in API (American Petroleum Institute) units, which makes a common standard for log comparison. The scale was chosen so that a value of zero would mean no radioactivity and a value of 100 would match a typical Mid-continent shale. In practice, shales can be somewhat variable in their radioactivity according to their silt content, types of clay mineral, and the occurrence of small amounts of uranium.

Figure 2. Use of gamma-ray log for stratigraphic subdivision of Dakota Aquifer in KGS Jones #1 NENENE 2-10S-8W, Lincoln County, Kansas.


Once the stratigraphic boundaries of the Dakota Aquifer are located (Figure 2), the gamma-ray log can be used to mark off depth intervals of sandstones and shales. As a general rule-of-thumb, experience has shown that a value of 60 API units is a satisfactory boundary to differentiate sandstones (below 60) and shales (above 60). An example of using this procedure is shown in Figure 3. The subdivision of a Dakota Aquifer section into sandstones and shales reveals the structure of aquifer and aquitard layers.

Figure 3. Use of gamma-ray log to subdivide the Dakota Aquifer in KGS Jones #1 between sandstone aquifer zones and shale aquitard zones theough the use of a gamma-ray cut-off value.


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Kansas Geological Survey, Dakota Aquifer Program
Updated April 9, 1996
Scientific comments to P. Allen Macfarlane
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