device may, for instance, be good for marking the thin breaks but may not 

 give an exact representation of the thick layers, whereas another device that 

 permits the estimation of fluid content may not be suitable for locating formation 

 boundaries. 



In early electric logging, a single resistivity curve was used. Later it was 

 found best to use three different electrode arrangements together in order that 

 the resistivity curves recorded may give as complete as possible a picture of 

 all the formations encountered in the borehole, however different their character- 

 istics may be. 



The three resistivity curves of the conventional log are run respectively 

 with a short normal device (AM = 16 inches), a long normal (AM = 64 

 inches) , and a long lateral (AO = 18 feet 8 inches) . 



This combination of three curves with three different devices, however, 

 does not always give all desirable information. Theoretical considerations and 

 field experience have shown that using one or several other devices of the 

 conventional type in addition to the standard ones does not help appreciably. 

 A more complete and accurate definition of beds and a closer investigation of 

 reservoir characteristics necessitate the application of the new methods described 

 later (MicroLog and resistivity methods using focusing systems) . 



Curve Shapes - Laboratory Results 



Figure 14-11 shows the curve recorded in the laboratory with normal 

 devices for homogeneous resistive layers sandwiched between beds of low 

 resistivity. The point of measurement of the readings is a point midway be- 

 tween A and M on the sonde. 



Obviously, the curves are symmetrical with respect to the center planes of 

 the layers. This is a general feature of the normal device. As a matter of fact, 

 the same curves are recorded if M is above A instead of A above M as indicated 

 in the figure. 



The upper part of the figure illustrates a bed thicker than the spacing 

 (bed thickness e — 6d; spacing AM — 2d\ d — hole diameter). It is 

 observed that the boundaries of the bed are not sharply indicated on the resistivity 

 log but tend to be rounded off owing to the influence of the drill hole. More- 

 over, the thickness of the bed, as indicated by the distance between the two 

 points of inflection P and P', on the curve, is less than its actual thickness by 

 an amount equal to the spacing. The error in picking the boundaries of thick 

 resistive beds is small for normal curves of short spacings, and this is one 

 reason for the recording in practice of a short normal. It should be remember- 

 ed, however, that normal curves tend to show resistive beds thinner than they 

 actually are by an amount equal to the spacing. In a similar manner they tend 

 to show conductive beds thicker than they actually are by an amount equal 

 to the spacing. 



288 



