This circumstance (which is very frequent in the Gulf Coast, for example) 

 seems to be due to the difference in specific gravities between the mud filtrate 

 and the connate water which permits the segregation of the two fluids. This 

 is more likely to take place in a gas-bearing formation. 



The invasion may also be extremely shallow and even negligible in low 

 water-loss mud or oil-base mud. 



Uncontaminated Zone 



The resistivity of the uncontaminated zone of the formation beyond the 

 invaded zone is called the true resistivity, Rj. The measuring devices do not 

 always give the value of the true resistivity directly. Other media, such as mud 

 column, invaded zone, and adjacent formations, may have a strong influence 

 on the measurements. The values recorded are, therefore, apparent resistivities 

 (symbol R a ), generally different from the true resistivities. 



The water saturation in the uncontaminated zone is given by equation 5, 

 repeated below: 



( F x R w \ 



(3) 



with S w = the water saturation in the uncontaminated zone ; F = the formation 

 factor; R w = the resistivity of the interstitial (connate) water; and R^ = the 

 true resistivity of the bed under study. 



On Figure 14-5B the resistivity is represented as decreasing outwardly from 

 Rj; to K t in a water-bearing sand. This feature is general since the mud filtrate, 

 R m /, is usually more resistive than the connate water, R w . For an oil-bearing 

 formation, the curve is shown going up from K x0 to R t ; actually, the reverse 

 trend (R^ >Ri) is also very frequent wherever R m/ is considerably greater 

 than R^. 



INSTRUMENTS, METHODS, The standard Electric log, Microlog, Induction 

 AND THEIR log, Laterolog, and Microlaterolog are dis- 



APPLICATIONS cussed in the following sections. 



Standard Electric Log 



The standard electric log of today consists of two basic types of record- 

 ings : on the left side of the log appears the spontaneous potential of the 

 borehole; the right side of the log is devoted to recordings of the electrical 

 resistivities of the formations as encountered in the borehole. 



There are various systems of measuring resistivities, but those most 

 commonly employed are multi-electrode normal and lateral resistivity record- 



276 



