400 Subsurface Geologic Methods 



against the wall of the drill hole. Under those conditions, the system 

 measures the average resistivity of the small volume of material — here- 

 inafter referred to as a "microvolume" — which is located under the pad, 

 and which is, therefore, electrically shielded against the short-circuiting 

 action of the mud. Two di£ferent electrode systems, with different depths 

 of investigation, are generally used in combination to provide two logs 

 that are recorded simultaneously. For both electrode systems, the spac- 

 ings are very small — usually one inch or two inches. In the discussion, 

 the systems which have the smallest and the largest depth of investigation 

 are respectively referred to as the "short spacing" and the "long spacing." 



When the pad is applied to a permeable bed, the mud cake repre- 

 sents a substantial porportion of the microvolume. Inasmuch as the mud 

 cake has a resistivity Rmc which can be estimated to be only about twice 

 the resistivity R^, of the mud, the resistivities recorded through microlog- 

 ging — hereinafter referred to as micro-resistivities — are never very high 

 opposite permeable beds, and are appreciably related to the resistivity of 

 the mud. The other part of the microvolume is constituted by a fraction 

 of the solid structure of the permeable bed whose pores are almost com- 

 pletely filled by the mud filtrate. The resistivity of that part of the micro- 

 volume is, therefore, not much different from the value F X R^ ^^ which 

 corresponds to complete mud filtrate saturation, so that it is also directly 

 related to the resistivity of the mud. 



It can easily be deduced from these considerations that the microre- 

 sistivities measured opposite a permeable bed cannot generally be higher 

 than a certain number of times the resistivity of the mud, unless the mud 

 cake is very thin, and unless, simultaneously, the formation factor F is 

 very high. A corresponding limit Rum can be set at about 20 or 30 times 

 the resistivity of the mud for the average case; therefore one of the rules 

 of interpretation for the microlog is to classify as most probably imperv- 

 ious all formations for which the microresistivities are higher than a 

 certain limit Rum directly related to the resistivity of the mud. 



Because of a smaller depth of investigation, the short spacing is more 

 influenced by the mud cake, and, therefore, generally gives a smaller ap- 

 parent resistivity than the long spacing. This difference between the 

 microresistivities recorded using two different depths of investigation is 

 called "departure," and is said to be positive when the longer spacing 

 gives the larger resistivity. When there is a large percentage of "positive 

 departure," the formation can almost certainly be interpreted as perme- 

 able. 



When the pad is applied to an impervious bed of low resistivity, 

 both spacings measure substantially the same resistivity, which is that of 

 the formation, and there is no appreciable departure between the micro- 

 resistivity curves. If the resistivity of the impervious bed is very high, 

 the microresistivities can differ appreciably from the formation resistivity, 



^^ Archie, G. E., The Electrical Resistivity Log as an Aid in Determining Some Reservoir Character- 

 istics: Petroleum Technology, vol. J, 1942. F is the formation factor. 



