Origin and Sources of GR

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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.
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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.

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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.

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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.

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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

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1 µ gm Ra-eq / Ton 16.5 ?API unit
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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
  cased-hole 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.

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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
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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.
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Gamma Ray and Caliper logs Correlation
Gamma Ray and Caliper logs Correlation
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Gamma Ray and Caliper logs Correlation
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Gamma Ray, SP, and Caliper logs Correlation
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Vsh (shaliness Calculation)
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Vsh Estimation from Gamma Ray
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Vsh Estimation from Gamma Ray
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Units
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For one type of tool, the relationship between
the concentration of the three radioactive
components and the total gamma ray signal in API
units (?API ) is given approximately by ?API 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 ?API unit