the effects of the borehole and of the adjacent formations. Since the diameter 

 of invasion generally is not known, the correction for its influence requires 

 one additional measurement by another suitable device. The short normal can 

 be used for this. The value of R* can then be derived from the readings of the 

 induction log and of the short normal by means of interpretation charts. In 

 this procedure, the knowledge of R x0 , the flushed zone resistivity, is necessary 

 (Rxo is determined by the MicroLog or the MicroLaterolog ; it can also be 

 derived from the value of porosity obtained from other sources). These charts, 

 at the same time, give a good estimate of the depth of invasion. 



If the value of R* cannot be determined exactly because the invasion is 

 too deep and/or the bed is too thin, the approximation obtained from the 

 induction log is most often sufficient to provide a sound basis for the estimation 

 of fluid content, at least qualitatively. 



Field Examples 



Figure 14-15 is a composite example showing the induction log, the con- 

 ventional resistivity logs, and the MicroLog run in unconsolidated sand-shale 

 series, including a few hard streaks (Gulf Coast). 



The ability of the induction log to delineate the variations of resistivities 

 within highly conductive sections is clearly indicated over sections E and H. 



The advantage of induction logging over the conventional log to determine 

 true formation resistivities, particularly opposite thin beds, is also illustrated by 

 this example. Considering bed A, the thickness of which is about 8 feet accord- 

 ing to the MicroLog, it is obvious that the long normal reading is influenced 

 by the more resistive adjacent formations (shales: resistivity 1.5 to 1.8). The 

 short normal reading is somewhat higher because of an invaded zone, which 

 is more resistive than the uncontaminated zone (the mud resistivity is 1.1 at 

 BHT) . Furthermore, bed A is located within the blind zone of the lateral curve 

 due to a thin resistive streak above; and the reading is much too low (less 

 than 0.5). Finally, there is no doubt that the induction log reading (0.8) is 

 closest to the true values. 



Over intervals B and C, the long normal curve is distorted by the thin 

 highly resistive streak in between. As the thickness of this streak is smaller 

 than the spacing (i.e., 64 inches), the curve shows a depression at its level; and 

 the resistivities recorded just above and below (i.e., precisely opposite the two 

 beds B and C under investigation) are spuriously increased. The long normal 

 reads 1.5 and 1.8 ohms, respectively, opposite the two beds, whereas the in- 

 duction log readings, which are practically unaffected by the hard streak, are 

 0.8 and 0.7. Here again the short normal curve is influenced by the invasion, 

 and bed C is located in the blind zone of the lateral curve. Similar remarks can 

 be made for the other thin beds in the section, such as D, F, G, and I. 



303 



