Subsurface Logging Methods 379 



formations. The first condition, with a normal curve, is achieved best by 

 a short electrode spacing, AM; the second, by using a longer electrode 

 spacing in order to minimize the effects of the drilling fluid, the diameter 

 of the hole, and the invaded zone. As a result two or more resistivity 

 curves, one with a short spacing and others with a somewhat longer 

 spacing, are commonly recorded and presented on each log. 



The particular behavior of normal curves in resistive beds of various 

 thicknesses is illustrated in figure 163. It is to be noted especially that a 

 resistive bed equal to the electrode spacing gives no indication; a resistive 

 bed where thickness is less than the electrode spacing is shown as an in- 

 verted anomaly. 



The lateral-type curve, with the spacings commonly employed, is 

 usually adequate to minimize the eiOfect of the invaded zone and, at the 

 same time, to indicate the position of resistive zones. Figure 164 indicates 

 the behavior of the lateral curve with resistive beds of different thicknesses. 

 Note that it shows beds of all thicknesses, but that the top boundary of 

 formations whose thickness is greater than the AO spacing is indefinite, 

 and true values are shielded out for a distance equal to the AO spacing. 

 The actual thickness of beds less than AB is exaggerated by an amount 

 equal to the distance between the current electrodes. Below thin resistive 

 beds an abnormally low resistivity is measured for a distance equal to the 

 AO spacing regardless of the nature of the forma-tion opposite the section 

 (fig. 165). 



Application of the Electric Log 



Correlation — The utility of the electric log in detailed structural pool 

 studies or in general stratigraphic investigations is well known. Figure 166 

 is a typical correlation study in the Midcontinent area. 



Distinction between Porous and Permeable Formations and Non- 

 porous and Nonpermeable Formations — The spontaneous potential curve 

 usually indicates permeable formations containing saline interstitial water 

 by a marked negative anomaly. This characteristic is common for sands as 

 well as limestones or dolomites and for shallow as well as deep forma- 

 tions. Formations containing fresh interstitial water, on the other hand, 

 are usually indicated by their lack of SP anomaly or by a positive 

 anomaly. 



An electric-log analysis through a limestone or cemented-sandstone 

 section, where permeable zones occur interbedded with otherwise nonper- 

 meable beds, needs particular mention. The permeable zones, whether oil- 

 or water-bearing, are usually more conductive (less resistive) than the sur- 

 rounding nonpermeable formations because of the saline interstitial water. 

 The resistivity log exhibits, therefore, a lower value across the permeable 

 zones than across the nonpermeable ones (fig. 167) . The anomalies on 

 the SP log spread above and below the permeable beds to such an extent 

 that permeable-zone boundaries are not determined easily by a cursory ex- 



