Petrophysics knowledge_Radioactivity Spectral Gamma Ray.pdf

Basic knowledge for Radioactivity in spectral gamma ray equipment

1. BERKAT Sabrine
GeoScience Engineer
PETROPHYSICS
Spectral Gamma RaySpectral Gamma Ray

2. designated as SGR, is a diagraphic technique employed to
quantify the natural gamma radiation spectrum emitted by
geological formations.
Spectral Gamma Ray
BERKAT Sabrine_GeoScience Engineer

3. By integrating this data with
additional diagraphic
measurements, one can
delineate the mineralogical
composition of clays, such as
kaolinite, chlorite, illite, and
montmorillonite. Furthermore,
this method facilitates the
identification and
quantification of thorium- and
uranium-bearing heavy
minerals.
Spectral
Gamma Ray
© Weatherford
Logging tool
BERKAT Sabrine_GeoScience Engineer

4. SGR LOG
The Spectral Gamma Ray log comprises three distinct curves
representing thorium (ppm), uranium (ppm), and potassium (%). In
contrast, the Gamma Ray log encompasses two curves: the
Standard Gamma Ray and the Computed Gamma Ray (CGR).
The SGR records the cumulative gamma radiation from all
sources, while the CGR is calculated by excluding the uranium
contribution from the SGR. Consequently, the CGR, which
reflects thorium and potassium sources, functions as an
enhanced tool for evaluating clay content, free from uranium
interference.
© Serra et al 1980
BERKAT Sabrine_GeoScience Engineer

5. Thorium/Potassium Ratio
The thorium-potassium crossplot helps identify and
differentiate clay minerals. It uses Th/K ratios,
calculated from thorium (ppm) and potassium (%), to
indicate potassium enrichment relative to thorium.
The crossplot should be used with care due to
the compositional variability of clay minerals.
Illite shows higher
potassium than
other clays, while
kaolinite shows
minimal
potassium.
© Serra et al 1980
BERKAT Sabrine_GeoScience Engineer

6. The Thorium-Uranium (Th/U) ratio has proven
useful in identifying "geochemical facies."
The Th/U ratio serves as an indicator of redox
potential. Uranium, in its tetravalent, insoluble
form, is fixed under reducing conditions but can be
converted into a soluble hexavalent state that is
mobile in solution. In contrast, Thorium exists only
in a tetravalent, insoluble form, making it a useful
standard for comparison.
Thorium/Uranium Ratio
BERKAT Sabrine_GeoScience Engineer

7. Thorium/Uranium Ratio
Adams and Weaver's 1958 study found that the
Th/U ratio correlates strongly with depositional
environments.
A Th/U ratio below 2 suggests a reducing, marine
environment favorable for uranium fixation,
while a ratio above 7 indicates uranium
mobilization, pointing to an oxidizing, possibly
terrestrial environment.
© Adams and Weaver 1958
BERKAT Sabrine_GeoScience Engineer

8. Main Application of SGR
Identification of lithology, facies, and
depositional environment.
Depth Correlation.
Geochemical and Mineralogical
study.
BERKAT Sabrine_GeoScience Engineer

9. Helps interpret depositional environments by analyzing
the shape of the curve to deduce grain size fluctuations
and sedimentary facies changes. Bell-shaped curves
often indicate upward-coarsening sequences, typically in
sandstone/clay formations.
Applocation of SGR
01
Identification of lithology, facies,
and depositional environment.
BERKAT Sabrine_GeoScience Engineer

10. Discontinuities may lead to phosphate nodule
accumulation, visible as U-shaped peaks in logs. High U
values and low Th/U ratios can signal condensed marine
sequences, while Th/U ratios assist in identifying
transgressive and regressive intervals.
Applocation of SGR
01
Identification of lithology, facies,
and depositional environment.
BERKAT Sabrine_GeoScience Engineer

11. Applocation of SGR
02
Geochemical and
Mineralogical study
The concentrations of the three main radioactive
elements in a formation can often provide insights
into its mineralogy and/or geochemistry.
For example, high Th values may indicate the
presence of heavy minerals, particularly in channel
sand deposits overlying an erosion unconformity.
Increased Th values can also be associated with a
higher proportion of terrigenous clay (Hassan et al.,
1976).
BERKAT Sabrine_GeoScience Engineer

12. Applocation of SGR
02
Geochemical and
Mineralogical study
Elevated Uranium levels are frequently linked to
organic matter. For instance, particularly high
Ur concentrations (>5 ppm) and low Th/U ratios
(<2) are found in black shale deposits (Adams
and Weaver, 1958).
In sandstones, high K values can be attributed
to the presence of potassium feldspar or micas
(Humphreys & Lott, 1990; Hurst, 1990).
Glauconite typically produces a very distinctive,
almost diagnostic peak in the K log.
BERKAT Sabrine_GeoScience Engineer

13. Applocation of SGR
03
Depth Correlation
The SGR log primarily reflects fluctuations in the
formation's mineralogy rather than physical
properties such as lithification. It is especially
useful for making regional inter-well
comparisons between major stratigraphic units.
BERKAT Sabrine_GeoScience Engineer

14. Identification of Silurian
Reservoir Clay-sandstone and the
Silurian Source Rock in the Berkin
Basin, using Spectral GammaRay
Case Study
Final Year Project Thesis
by BERKAT Sabrine & BEN ACHOUR Lina
Supervised by KERDJIDJ M.Kamel
BERKAT Sabrine_GeoScience Engineer

15. Case Study
AbstractIn the stratigraphic series of the Berkine Basin, located in southeastern Algeria, the terminal Silurian is marked by a monotonous
silico-clastic sequence composed of alternating clayey, sandstone, and silty layers overlying the underlying Silurian clays. This clayey-
sandy sequence is part of the lower section of the SAG reservoir, which is divided into several units. It is characterized by significant
variations in thickness and facies that are challenging to distinguish using total gamma ray measurements. To overcome this
difficulty in facies characterization, we employed gamma ray spectrometry.
In the stratigraphic series of the Berkine Basin,
located in southeastern Algeria, the terminal
Silurian is marked by a monotonous silico-clastic
sequence composed of alternating clayey,
sandstone, and silty layers overlying the
underlying Silurian clays. This clayey-sandy
sequence is part of the lower section of the SAG
reservoir, which is divided into several units. It is
characterized by significant variations in thickness
and facies that are challenging to distinguish
using total gamma ray measurements. To
overcome this difficulty in facies characterization,
we employed gamma ray spectrometry.
BERKAT Sabrine_GeoScience Engineer

16. Case Study
The SGR had been used to :
Differentiate between
clay and sandstone, both
of which show a GR
reading of 100 API,
Identify the Source Rock
Correlation,
Distinguish between
Silurian and Triassic
formations, and locate
the Hercynian
unconformity.
BERKAT Sabrine_GeoScience Engineer

17. In conclusion, the Spectral Gamma Ray log,
incorporating measurements of thorium,
uranium, and potassium, as well as the Th/K
and Th/U ratios, provides critical insights
into mineralogy and depositional
environments. This log enables accurate
identification of clay minerals, heavy
minerals, and sedimentary facies, while also
elucidating redox conditions.
In conclusion, the Spectral Gamma Ray log,
incorporating measurements of thorium,
uranium, and potassium, as well as the Th/K
and Th/U ratios, provides critical insights
into mineralogy and depositional
environments. This log enables accurate
identification of clay minerals, heavy
minerals, and sedimentary facies, while also
elucidating redox conditions.
BERKAT Sabrine_GeoScience Engineer

18. References
Cours, diagraphie petrolieres, IFP par
professeur D. Chaplier
SERRA O: Diagraphie, Acquisitions et
Application. Edition Serra log, 2000.
SERRA O : “Diagraphies Differes base de
l’interpretation” Memoire 7 Tome2. Etudes
et productions Schlumberges, Montrouges
1985.
SERRA.O “Diagraphies Differes base de
l’interpretation” Memoire 1 Tome 1. Services
techniaues Schlumberger, Paris 1979.
BERKAT Sabrine_GeoScience Engineer

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GeoScience Engineer
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