Formation Evaluation Through Casing - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 185

Table 11.10 Geological significance of natural gamma ratios
Ratios Remarks

Thorium/
Uranium (Th/U)

In sedimentary rocks, Th/U varies with depositional environment Th/U
>7: continental, oxidizing environment, weathered soils, etc.
<7: marine deposits, gray and green shales, graywackes
<2: marine black shales, phosphates
In igneous rocks, high Th/U indicative of oxidizing conditions by magma
Before crystallization and/or extensive leaching during
Postcrystallization history
Source rock potential estimates of argillaceous sediments (shales)
Major geologic unconformities
Distance to ancient shore lines or location of rapid uplift during time of
deposition
Stratigraphic correlations, transgression vs. regression, oxidation vs.
reduction regimes, etc.

Uranium/
Potassium (U/K)

Source rock potential of argillaceous sediments
Stratigraphic correlations
Unconformities, diagenetic changes in argillaceous sediments,
carbonates, etc.
Frequent correlation with vugs and natural fracture systems in subsurface
formations, including localized correlation with hydrocarbon shows on
drilling mud logs and core samples both in clastic and carbonate
reservoirs

Thorium/
Potassium (Th/K)

Recognition of rock types of different facies
Paleographic and paleoclimatic interpretation of facies characteristics
Depositional environments, distance from ancient shore lines, etc.
Diagenetic changes of argillaceous sediments
Clay typing: Th/K increases from glauconite ⃒ muscovite ⃒
illite ⃒ mixed-layer clays ⃒ kaolinite ⃒ chlorite ⃒ bauxite
Correlation with crystallinity of illite, average reflectance power,
paramagnetic electronic resonance

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