An important exception to the second rule occurs when the invasion is 

 very shallow and the formation is salt water bearing. The depth of invasion 

 may be so small that the volume of formation involved in the resistivity meas- 

 urements includes a portion of the formation beyond the flushed zone. Under 

 such conditions the mud cake is more resistive than the formations immediately 

 behind, and the micronormal reading is lower than the microinverse. In other 

 words, the separation is negative. Shallow invasion is prevailing whatever 

 the type of mud when the porosity and vertical permeability are very high 

 and in fissured formations where the matrix is not flushed. Also, instances of no 

 separation can occur when the mud cake is fairly thin, especially in zones of 

 high porosity. 



It can be added that the effect of the mud cake is to level out the resistivity 

 readings, so that the curves never show sharp variations opposite a permeable 

 bed, even when the formation factor changes appreciably within the bed. 

 The same criteria apply in granular porosity, as well as in well-distributed 

 secondary porosity. 



The above discussion shows that the MicroLog can detect the permeable 

 beds even with very low permeability provided it is sufficient for the mud cake 

 to build up. Therefore, a bed may be indicated by the MicroLog as being 

 permeable, whereas the permeability is too low for commercial production. 



Tight Sections 



In tight sections the electrode system is separated from the formation 

 only by a thin mud film, maybe 1/16 inch thick or less. The MicroLog 

 readings accordingly are very high, at least equal to 20 times the mud 

 resistivity. The mud film, furthermore, does not have a constant thickness 

 because the Avail of the hole is not perfectly smooth. Depending on the depth 

 of the irregularities in the hole wall, the path offered to the current to escape 

 toward the mud column is variable. As a result, the curves show numerous sharp 

 peaks and depressions; and the separation may be positive or negative. 



Shales 



When there is no caving in the shales, the pad again is separated from the 

 formation by a thin mud film. The reading is equal to, or more often smaller 

 than, the resistivity of the shales. The separation may be negative, nil, or 

 slightly positive. This behavior of the MicroLog in shales is not entirely 

 explained. It is assumed that the occurrence of negative separation may be 

 due partly to the anisotropy of the shales. 



In practice, when a positive separation is observed opposite a shale, a 

 confusion is possible with a permeable bed. It is then indispensable to use 

 other curves to solve the ambiguity. In many instances the SP curve is sufficient. 

 Very often, however, these curves do not give clear enough indications 



296 



