In making the log, the bottom of the well casing will be detected when the probe enters it. The effect on
the curves will be that both fall off to extremely low values, 5 to 20 ohmft, and then remain fairly constant.
Where the casing is seated into very low resistive shale, it may be rather difficult to determine the exact
position of the casing by this method.
2-6.9 The SP Curve.
The spontaneous potentials measured in a borehole are of great value in deep oil wells where saline
waters are encountered. For these situations the SP curve exhibits a great deal of character and can be
related to relative changes in formation permeability.
When logging in fresh water horizons, the SP curve will usually be featureless and provide little or no