Environmental Engineering Reference
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Another technique is to apply the U, Th, and K concentrations in combination
with other log data to determine lithology and clay type, as described by Marett
et al. Still another approach could be called the geochemical method as described by
Hassan et al. ( 1976 ). Figure 11.11 illustrates the variation of the thorium/potassium
ratio in a number of minerals ranging from potassium-feldspar to bauxite.
Fig. 11.11 Thorium/Potassium ratios for various minerals. Reprinted by permission of the
SPWLA from Hassan et al. 1976. Courtesy Schlumberger
Figure 11.12 maps a number of radioactive minerals as a function of their
thorium and potassium content. Other elemental ratios are also useful indicators.
For example, a low U/Th ratio indicates reduced black shales. Uranium by itself
may indicate a high organic carbon content, which in turn may indicate the
presence of gas.
20
16
KAOLINITE
70% ILLITE LINE
12
MIXED LAYER CLAYS
8
ILLITE
MICAS
40% MICAS LINE
80% GLAUCONITE LINE
30% FELDSPAR LINE
4
GLAUCONITE
Th/K = .5
FELDSPAR
0
K(%)
0
1
2
3
4
K (%)
Fig. 11.12 Thorium/Potassium crossplot for minerals identification. Courtesy Schlumberger
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