Page 72 - Basic Well Log Analysis for Geologist

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RESISTIVITY LOGS

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Figure 21. Example Laterolog* and Microlaterolog*. The purpose for presenting this log is to iHustrate the log curves, and to give
you guidance on picking log values. These logs are used when Ry = Ry.
Track #1—The log track on the far left in this example is a gamma rav log. Gamma ray logs are discussed ina later
chapter. but they commonly accompany Laterologs*.
Track #2—The middle log track here ts the Laterolog* which measures the deep resistivity (R,) or true resistivity of the
formation. Note that the scale increases from left to right, in increments of 5 ohm-meters from 0 to 50 on the first
eycle. and in hybrid increments from 0 to * on the second cycle.
At the sample depth of 3.948 ft the Laterolog* value reads 21 ohm-meters.
Track #3—The right-hand log in this suite is the Microlaterolog* which measures the resistivity of the flushed zone (R,,).
Note that the scale starts with zero between tracks #2 and #3—that is, zero for the Microlaterolog® ts not the
same point as zero for the Laterolog* farther to the left. The seale ranges from 0 to 50 ohm-meters in increments
of 5 ohm-meters. There is no second cycle recorded.

At the sample depth of 3.948 ft the Microlaterolog* reads 10 ohm-meters. or the depth line intersects the log curve
at two increments from zero.
Note: In order to correct (for invasion) the Laterolog* to true resistivity (R,). do the following (use the example at 3.948
ft ):
R, = 1.67(R,,) — 0.67 (R,,) (Hilchie, 1979)
R, = 1.67 (21) — 0.67 (10)
R, = 28.4 ohm-meters
Where:
R, = resistivity of the uninyaded zone
Ry = Laterolog* resistivity (21 ohm-meters at 3,948 ft )
R,, = Microlaterolog* resistivity (10 ohm-meters at 3,948 ft )