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
Web comments to webadmin@kgs.ku.edu
URL=http://www.kgs.ku.edu/Dakota/vol1/petro/petro03.htm