16.5.4.2 Uses of Gamma and Spectral
Gamma Ray Logs
The gamma ray is a simple but very useful log. High
vertical resolution makes it particularly well suited for
depth matching and fine-scale correlation. The other
most common uses of gamma and spectral gamma ray
logs are (1) lithology discrimination, (2) shale volume
estimation, and (3) analysis of facies and depositional
environment.
Lithology Discrimination
A prime application of the gamma ray log is for discrimination of different lithologies (Fig. 16.12). While it cannot uniquely define any lithology, the information it
provides is invaluable when combined with information
from other logs. Shales, organic-rich shales and volcanic
ash show the highest gamma ray values, while halite,
anhydrite, coal, clean sandstones, limestone and dolomite
have the lowest values. This difference in radioactivity
between shales and sandstones/carbonates allows the
gamma tool to distinguish between shales and non-shales.
Care must be taken not to overgeneralise these rules. For
example a clean sandstone may contain feldspars (arkosic
sandstones), micas (micaceous sandstones) or both
(greywackes), or glauconite, or heavy minerals, any of
which will give the sandstone higher gamma ray values
than would be expected from a pure quartzitic sandstone.
Black shales (e.g. hydrocarbon source rocks) in
particular, with their substantial organic content, produce marked responses on the gamma ray log because
they normally have a higher uranium content than
other shales. The Kimmeridge (Upper Jurassic) shale
in the North Sea contains 2–10 ppm uranium and is
often referred to as the “hot shale”. Normally shales
contain <1 ppm uranium, but 10–12 ppm thorium,
which represents ~50% of the total radioactivity.
Limestones have very low concentrations of U, Th
and K, and give very low gamma-ray responses. In
evaporite sequences, however, gamma logs are very
sensitive indicators of potassium salts.
Fig. 16.11 Some typical responses of gamma ray and spectral gamma ray logs against different lithologies. The spectral gamma
ray responses show individual contributions of thorium, potassium and uranium to the overall radioactivity
16 Well Logging: Principles, Applications and Uncertainties
397
Gamma Ray Logs
The gamma ray is a simple but very useful log. High
vertical resolution makes it particularly well suited for
depth matching and fine-scale correlation. The other
most common uses of gamma and spectral gamma ray
logs are (1) lithology discrimination, (2) shale volume
estimation, and (3) analysis of facies and depositional
environment.
Lithology Discrimination
A prime application of the gamma ray log is for discrimination of different lithologies (Fig. 16.12). While it cannot uniquely define any lithology, the information it
provides is invaluable when combined with information
from other logs. Shales, organic-rich shales and volcanic
ash show the highest gamma ray values, while halite,
anhydrite, coal, clean sandstones, limestone and dolomite
have the lowest values. This difference in radioactivity
between shales and sandstones/carbonates allows the
gamma tool to distinguish between shales and non-shales.
Care must be taken not to overgeneralise these rules. For
example a clean sandstone may contain feldspars (arkosic
sandstones), micas (micaceous sandstones) or both
(greywackes), or glauconite, or heavy minerals, any of
which will give the sandstone higher gamma ray values
than would be expected from a pure quartzitic sandstone.
Black shales (e.g. hydrocarbon source rocks) in
particular, with their substantial organic content, produce marked responses on the gamma ray log because
they normally have a higher uranium content than
other shales. The Kimmeridge (Upper Jurassic) shale
in the North Sea contains 2–10 ppm uranium and is
often referred to as the “hot shale”. Normally shales
contain <1 ppm uranium, but 10–12 ppm thorium,
which represents ~50% of the total radioactivity.
Limestones have very low concentrations of U, Th
and K, and give very low gamma-ray responses. In
evaporite sequences, however, gamma logs are very
sensitive indicators of potassium salts.
Fig. 16.11 Some typical responses of gamma ray and spectral gamma ray logs against different lithologies. The spectral gamma
ray responses show individual contributions of thorium, potassium and uranium to the overall radioactivity
16 Well Logging: Principles, Applications and Uncertainties
397