Page 77 - The Geological Interpretation of Well Logs

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THE GAMMA RAY AND SPECTRAL

GAMMA RAY LOG

7.1 Generalities while the spectral gamma ray log shows the amount of each
individual element contributing to this radioactivity.
The log The geological significance of radioactivity lies in the
The gamma ray log is a record of a formation’s radioactivity. distribution of these three elements. Most rocks are
The radiation emanates from naturally-occurring uranium, radioactive to some degree, igneous and metamorphic
thorium and potassium (see below). The simple gamma ray rocks more so than sediments. However, amongst the
log gives the radioactivity of the three elements combined, sediments, shales have by far the strongest radiation. It is
GAMMA RAY LOG SPECTRAL GAMMA A RAY
A
(natural radioactivity) U een
Scale: API units Th ppm oe K %
oe 7, 80, 120, 160, 200 ogi ott
ee

SHALE

compact f.-
SANDSTONE ~---£ Th
porous

LIMESTONE

ARKOSE “U UK

MICACEOUS

SANDSTONE

glauconitic
;
SILT/FINE SAND bands
Heavy mineral
KAOLINITE I
Pure
MONT - clay
MORILLONITE] .
mineral
ILLITE

ORGANIC RICH/
BLACK SHALE
COAL

FINING-UP — Shaly
SANDSTONE
Clean

SHALE
SHALE BASE
ae LINE
Figure 7.1 The gamma ray log and spectral gamma ray log: some typicai responses. The gamma ray log shows natural radioactivity.
The spectral garama ray log gives the abundances of the naturally radioactive elements, thorium, Th and uranium, U in parts per
million (ppm) and potassium, K in %. F - feldspar, M = mica, * = glauconite.
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