Page 146 - Basic Well Log Analysis for Geologist

LITHOLOGY LOGGING AND MAPPING TECHNIQUES

Figure 49. Example gamma ray log depicting a 20 API unit gamma ray cutoff used to determine a clean carbonate interval.
Given: The volume of shale (V,,) cutoff is arbitrarily set at 5% (V,, = 0.05). Next, determine the garnma ray index from
the chart in Figure 38; Chapter V (gamma ray index where V,, = 5%. is Igrz = 0.10).
Procedure:
1. Determine gamma ray cutoff (see log; and Fig. 38).

Remember:
Igr = See SR
Rinax ~ GRnin
Where:
GRiog = gamma ray log
GRyax = gamma ray maximum (shale)
GRypin = gamma ray minimum (shale-free sandstone or carbonate)
From Log:
GRyax = 90 (from shale zone on log)
GRmin = 12 (from clean carbonate zone on log)
Iga = 0.10 (gr for Vn = 0.05; given)
Then:

_ GR, — GRnin
Icr ~ AB
GRinax ~ GRain
Or:

0.10 = Ree —~ 12 12
90 — 12
Therefore:

GRjog = 19.8 (round off to 20 API units)
2 20 API represents clean carbonate where the volume of shale is equal to (or less than) 5%.
Draw a vertical line from the scale value of 20 API units and determine the thickness and limits of the clean
carbonate formation (bioherm) much as you determined alpha values in Figure 48.

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