In interpretation, the unknowns will generally be 1, 2, 3 and 4. Granular materials will be high resistive
compared to fines such as silt and clay; crystalline materials (such as limestone or granite) will be high
resistive compared to the granular materials.
The quality of the formation water will greatly affect the measured resistively.
In general, the resistively of a formation will vary in an inverse proportion to the total dissolved solids. For example, all
other conditions remaining the same, if the total solid content increases, the formation resistively will decrease. Hence a
clean sand filled with salty water may actually be extremely low resistive.
Porosity of the formation also has an effect on the resistively. It is not as pronounced as the effect from
water quality. In the logging of chemical precipitates, such as limestone, changes in porosity may enable
you to detect the water producing zones. Increased porosity will lower the formation resistively and hence
in such material a low resistive zone (where no shale is present) is indicative of increased porosity. This is
then indicative of possible water production.
The exact range of values for clean sand, gravel, or sandstone is something which you learn by experience
in your own particular area. In the midwest United States, clean sand and gravel generally exhibit
resistively values in the range of from 350 to 1000 ohmft. The lower values apply to formations having
water quality in the range of 300 to 400 ppm total solids and the upper values apply for formation waters
having 100 to 150 ppm total solids. The above remarks are, of course, very general and are included for
2-6.6 Selecting Formation Contact. In "picking" the formation boundaries, the 0.25 ft curve should be used wherever
possible. The inflection point (the point midway between changes in curvature of the resistively curve) of the resistively
curve is used to mark the contact between different formations.
2-6.7 Correlation by Electrical Logs.
A useful application of the electrical logs is in correlating formation thickness and depths from one well to
another. For example, two wells within a few feet of each other invariably will give identical electrical logs.
When the wells are farther apart, the correlation will still be recognizable and the changes which do occur,
as for example thickening or thinning of beds, are exactly the information needed to guide further
Correlation is commonly possible to considerable distances in bedrock formation, in the order of thousands
of feet. Because of the variable nature of unconsolidated glacial and alluvial deposits, do not expect such
distances except in special cases of a single, widespread type of deposit.
2-6.8 The Effect of Metal on the Resistively Log.
Because metal is such a good conductor, its presence in the zone of measurement, as for example air
lines which have dropped to the bottom of the well, will cause a major decrease in the resistively and make
the log unusable, in so far as determining formation type. This effect, however, may be used to locate
such steel in the well.