Page 78 - Basic Well Log Analysis for Geologist

RESISTIVITY LOGS

Figure 24. Example Microlog* with spontaneous potential! log and caliper. This log demonstrates permeability two ways: positive
separation between the micro normal and micro inverse logs in tracks #2 and #3 and decreased borehole size due to
mudcake, detected by the caliper log in track #1.
Examine the log from a sample depth 5, 146 ft to 5,238 ft.
Track #1—Note that the caliper shows a borehole diameter of approximately I 1 inches just above the sample depth, but
the hole size decreases to about 8.5 inches within the sample interval (the caliper measurement is shown by the
solid line in track #1). thus indicating the presence of mudcake and a permeable zone.
Track #2—Note the positive separation between the micro normal log and the micro inverse log: the separation is about 2
ohm-meters. Positive separation is tndicated where the resistivity value of the micro normal log (shown by the
dashed line) is greater than the resistivity value for the micro inverse log (shown by the solid line).

This higher micro normal resistivity value is because the micro normal curve reads deeper into the flushed zone.
The combination of mud filtrate, formation water and/or residual hydrocarbons and rock in the flushed zone gives
a higher resistivity reading than the mudcake (measured by the micro inverse curve).

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