Origin and Sources of GR
Gamma Rays are bursts of high-energy electromagnetic
waves that are emitted spontaneously by some radioactive
elements ( unlike Alpha and Beta rays of low energy )
Correlation curves (Lithology curves) :
Gamma Ray ( can substitute SP if not available ) and have
similar appearance to SP and compliment the SP.
Gamma Ray log measures natural radioactivity of formation
due to the radioactive material disintegration ( decay) :
– Potassium (K ) (isotope of atomic wt. 40)
– Uranium (U)
– Thorium (Th)
Indicates shale and clay present in sedimentary and igneous
rocks.
Statistical Variation phenomena
• GR emittion is not at constant rate and
speed, but at random, because it is a
natural phenomena. Thus, logging
speed should be slow to reduce this
effect.
GR Log Measuring Equipment
• GR sonde contains a detector to measure
Gamma radiation originating in the volume of
the formation near the sonde.
• They are two types:
– Geiger Mueller counters : used in the pasts
– Scintillation counters : now used for this
measurement; they are much more efficient than
the Geiger Mueller counters used in the past, and
they offer good formation detail.
Origin and Sources of GR, cont
• Radioactive elements tends to
concentrate in shale and clays due to
their structure and origin.
• Clean formation usually have very low
level of radioactivity, unless local
radioactive contaminants such as
volcanic ash or granite wash or organic
matter is present, or formation waters
contain dissolved radioactive salts.
Gamma Ray Log ; uses
•Gamma ray log is measurement of natural radioactivity in
formation verses depth.
•It measures the radiation emitting from naturally
occurring U, Th, and K.
•It is also known as shale log and as shale indicator.
•GR log reflects shale or clay content.
•Clean formations have low radioactivity level.
•Correlation between wells,
•Determination of bed boundaries, and sand count.
•Evaluation of shale content within a formation,
•Mineral analysis,
•Depth control for log tie-ins, side-wall coring, or WO,
perforating and completion.
•Used in tracer emitting and detection as in WI, GI,
cementing .
•Particularly useful for defining shale beds when the sp is
featureless.
•GR log can be run in both open and cased hole
1 µ gm Ra-eq / Ton = 16.5 γAPI unit
Gamma Ray Log Uses
• Generally, a gamma ray log indicates the amount of shale
present
• Low Natural Radioactivity
–
–
–
–
Sandstone
Limestones
Dolomites
Minimum reading at salt & anhydrite ( few API units)
• High Natural Radioactivity
– Shales, Mica , Feldspar, Volcanic, and NORM
• Gamma ray logs can be run in both openhole and casedhole environments.
• Although the overall reading of the gamma ray is affected
by the casing, the character of the log is not so that the
cased-hole gamma ray can be used to correlate with the
openhole gamma ray.
Gamma Ray Log & NGS
*Standard gamma ray logs measure the
total natural radioactivity of a formation.
*Information from cased hole logs:
- Gamma Ray Log - same as open hole
log;
*Natural Gamma Ray Spectrometry Log used to identify clay types and clay
volumes, and is a better shale indicator
than GR;
*The standard gamma ray log can be used
to calculate the fraction of the reservoir
that is shale by using a straight line
calculation
Gamma Ray Log & NGS
*Standard gamma ray logs measure the
total natural radioactivity of a formation.
*Special gamma rays logs called spectral
gamma rays can determine separately the
amount of gamma rays from potassium,
uranium, and thorium.
Although these logs are not commonly
run, they are often useful for the geologist
when trying to correlate various shales.
*The standard gamma ray log can be used
to calculate the fraction of the reservoir
that is shale by using a straight line
calculation.
Gamma Ray and Caliper logs Correlation
Gamma Ray
and Caliper
logs
Correlation
Gamma Ray and Caliper logs Correlation
Gamma Ray, SP,
and Caliper logs
Correlation
Vsh (shaliness Calculation)
Vsh Estimation from Gamma Ray
Vsh
Estimation
from Gamma
Ray
Units
For one type of tool, the relationship
between the concentration of the three
radioactive components and the total
gamma ray signal in API units (γAPI ) is
given
approximately by:
γAPI = 4 Th + 8 U + 16 K
when thorium (Th) and uranium (U) are
measured in ppm and potassium (K) in
percent by weight.
1 µ gm Ra-eq / Ton = 16.5 γAPI unit