Page 106 - Basic Well Log Analysis for Geologist

CHAPTER V

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GAMMA RAY LOGS

General
Gamma ray logs measure natural radioactivity in As an example of this calculation, pick these values from
formations and because of this measurement, they can be the gamma ray log in Figure 37 (they will be used in Figure
used for identifying lithologies and for correlating zones. 38):
Shale-free sandstones and carbonates have low
GRiog = 28 at 13,570 ft (formation reading)
concentrations of radioactive material, and give low gamma
GRpyin = [5 at 13,590 ft
ray readings. As shale content increases, the gamma ray log
GRmax = [28 at 13,720 ft
response increases because of the concentration of
radioactive material in shale. However, clean sandstone Then,
(t.e. low shale content) may also produce a high gamma ray
1, = 28 ot. 13
response if the sandstone contains potassium feldspars,
GR 128 — 15 113
nicas, glauconite. or uranium-rich waters.
Iog = 0.115
In zones where the geologist is aware of the presence of
potassium feldspars, micas, or glauconite, a Spectralog** Finally, the calculated value of the ganima ray index (IGgR) 1s
can be run in addition to the gamma ray log. The located on the chart in Figure 38. and a corresponding value
Spectralog** breaks the natural radioactivity of a formation for volume of shale (Vy) in either consolidated or
into the different types of radioactive material: (1) thorium, unconsolidated sands is determined.
(2) potassium, and (3) uranium. From Figure 38, and using a value for Ig of 0.115, find:
If a zone has a high potassium content coupled with a
V., = 0.057 older rocks (consolidated)
high gamma ray log response, the zone may not be shale.
Voy = 0.028 Tertiary rocks (unconsolidated)
Instead. it could be a feldspathic, glauconitic, or micaceous
sandstone. The volume of shale is also calculated mathematically
Besides their use with identifying lithologies and from the gamma ray index (Ig) by the following Dresser
correlating zones, gamma ray logs provide information for Atlas (1979) formulas:
calculating the volume of shale in a sandstone or carbonate.
Older rocks, consolidated:
The gamma ray log is recorded in track #1 (example, Fig.
37), usually with a caliper. Tracks #2 and #3 often contain Vi, = 0.33 [22 > Hor) — 1.0]
either a porosity log or a resistivity log.
or. Tertiary rocks, unconsolidated:
Volume of Shale Calculation Voy = 0.083 [237 * Iori — 1.0]
Because shale is more radioactive than sand or carbonate, Where:
gamma ray logs can be used to calculate volume of shale in Veh = Volume of shale
porous reservoirs. The volume of shale can then be applied Iop = gamma ray index
for analysis of shaly sands (see Chapter VI). Review - Chapter V
GRypay = Maximum gamma ray (shale) shale has high gamma ray readings. Shale-free sandstones
Calculation of the gamma ray index is the first step
needed to determine the volume of shale from a gamma ray |. Gamma ray logs are lithology logs that measure the
log tthe following formula from Schlumberger, 1974). natural radioactivity of a formation.
2. Because radioactive material is concentrated in shale,
GRiog ~ GRinin

lor =
— GR min
GRaax max
and carbonates, therefore, have low gamma ray readings.
Where: = gamma ray index (2) correlate between formations: and (3) calculate volume
3. Gamma ray logs are used to: (1) idenufy lithologies;
Isp
GR,» = gamma ray reading of formation
of shale.
GRyin = Minimum gamma ray (clean sand or carbonate)

Ot