When the depth of penetration of mud nitrate is very small, as in low- 

 water-loss mud and also in formations having a very high vertical permeability, 

 the value of R xo determined from microresistivity logs does not correspond 

 any more to the flushed zone only, but it may involve a part of the uncontam- 

 inated zone. The ratio R* /R«/ (corrected for residual oil saturation, if any) 

 does not, in this instance, represent the formation factor. Also, in the gas- 

 bearing formations, a great amount of gas may be present in the pores near 

 the wall of the hole and may affect the value of R xo to an extent that is difficult 

 to evaluate but that may be very great. 



Furthermore, with the present large sample taker, reliable determinations 

 of formation factor and porosity are possible from side wall cores. 



Hard Formations 



It seems that the basic tools for the determination of porosity should be 

 the MicroLaterolog and the neutron log. 



Another approach is based on the value of R*, the average resistivity of 

 the invaded zone, which can be estimated from the readings of the lime- 

 stone sonde or the short normal. This procedure, although it does not 

 provide such accurate and detailed results as the one based on R^ (when this 

 latter can apply) has proved valuable in many cases. 



Evaluation of Saturation 



Unconsolidated Formations 



The conventional electric logs may be used for the evaluation of saturation 

 when the beds are not too thin. In thin beds, the induction log is necessary. 

 It can be said that with induction logging a reasonable quantitative interpreta- 

 tion should be possible in most beds at least 5 feet thick. Reliable qualitative 

 results can still be obtained for thinner beds. 



Hard Formations 



Where wells are drilled with high-salinity mud, the salt-mud survey tech- 

 nique (gamma ray, Laterolog, MicroLaterolog, and/or neutron log) makes 

 possible a detailed investigation of porous and permeable sections. Quantitative 

 results can be obtained with a fair approximation down to a bed thickness of 

 4 or 5 feet; and, here also, good qualitative analysis is often possible for even 

 smaller bed thicknesses. 



The investigation of thin porous and permeable sections is more difficult 

 in this instance because appropriate induction logging and MicroLaterologging 

 tools are not yet standardized. 



It is, nevertheless, possible to obtain useful information on the average 

 value of saturation over thick intervals by using the average value of R t 



319 



