Page 66 - Basic Well Log Analysis for Geologist

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RESISTIVITY LOGS

Figure 18. Example of a Dual Induction Focused Log. Use this log to pick values and determine ratios for the tornado chart
exercise in Figure 19. The Dual Induction Focused Log is normally used when R,,¢ is much greater than R,. and also where
invasion is deep.
Track #1 in this log suite contains a gamma ray, SP, and R,,/R, quick look curves. The gamma ray. SP, and R,./R, quick
look curves will be discussed in subsequent chapters.
The resistivity scale in tracks #2 and #3 is a logarithimic scale from 0 to 2,000 ohm-meters. increasing from left to right.
Note the following logs.

Deep induction log resistivity—The dashed line ILD represents Ry 4 and measures the deep resistivity of the formation. or
close to true resistivity (R,). At the sample depth in this exercise (13,590 ft). deep resistivity (ILD) reads a value of
70,
Medium induction log resistivity—The dotted-and-dashed line ILM represents Ry ,, and measures the medium resistivity
of the formation, or resistivity of the invaded zone (R,). At the sample depth in this exercise (13,590 ft), medium
resistivity (R;) reads a value of 105.
Spherically Focused Log* resistivity—the solid line SFL* represents Rgpy* and measures the shallow resistivity of the
formation, or resistivity of the flushed zone (R,,). At the sample depth in this exercise (13,590 ft), resistivity of
the flushed zone (R,,) reads a value of 320.
The following ratios are needed for work on the tornado chart (Fig. 19), and the values are picked trom the
example log:
Reg /Ring = 320/70 = 4.6
Rim/ Rua = 105/70 = 1.5

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