that the potential at M is directly controlled by the resistivity of the bed. The 

 apparent resistivity then is close to the true resistivity of the bed. 



Figure 14-19 shows a typical example of a curve recorded in the labora- 

 tory opposite a thin noninvaded bed, more resistive than the adj acent formations, 

 and traversed by a borehole filled with mud of low resistivity. 



In this figure the thickness of the bed, e, is equal to five times the hole 

 diameter, d. The spacing of the Laterolog 7 device is 1.5c?. For a usual value 

 of hole diameter, say about 9 inches, these values correspond respectively to a 

 bed thickness slightly less than 4 feet and to a thickness of the current sheet 

 slightly greater than 1 foot. The resistivity of the bed is equal to 100, the 

 resistivity of the mud to 0.1, and the resistivity of the adjacent formations to 3. 



The curves recorded with the short normal AM = 1.75J (16"), a long 

 normal AM = Id (64"), and a lateral device AO = 2Sd (18' 8") are shown 

 on the figure for comparison. 



It is seen on the figure that the Laterolog device gives a much sharper 

 indication of the boundaries of the bed than the conventional devices. Moreover, 

 the value of the apparent resistivity read with the Laterolog on the center plane 

 of the bed is equal to 80, against 100; whereas the conventional devices show 

 4, 5, and 12, respectively. 



In hard-rock territories, the porous and permeable sections are most 

 often located between tight formations; in other words, the resistivity of 

 the reservoirs is usually smaller than the resistivity of the adjacent beds. The 

 result of the laboratory tests available for Rt<R s show that the effect of the 

 adjacent formations can be neglected without too great an error in saturation 

 evaluation if bed thickness is greater than about 4 feet. 



Conclusion 



The Laterolog gives a sharp record of the sequences of beds, whatever 

 the mud resistivity. It is, therefore, an excellent tool for formation definition 

 and for correlation. 



The Laterolog is chiefly appropriate to well logging in hard-rock ter- 

 ritories, where it has been accepted as essential for the definition of beds, for 

 correlation, and for reservoir evaluation. 



The Laterolog finds its most favorable conditions of applications in wells 

 drilled with high-salinity mud. Under such conditions, it constitutes the basic 

 tool for the investigation of R t and, hence, of saturation. 



In wells drilled with fresh mud, the Laterolog is generally used at the 

 present time as an addition to the conventional logs and the MicroLog. 

 The Laterolog may possibly be used in the future as an auxiliary tool to the 

 induction log. 



311 



