derived from the lateral and the average value of the formation factor derived 

 from the limestone sonde or the short normal. Such a procedure, which 

 involves several assumptions, has often provided valuable results. 



In hard formations permeability is generally low; and, as a result, a 

 resistivity gradient is observed on the logs between the zone of lowest water 

 saturation and the water level. The existence of this gradient is the basis 

 of a method for the delineation of intervals saturated with hydrocarbons. 



Investigations in Oil-Base Mud and Empty Holes 



The methods applicable in this instance are induction log, resistivity 

 curves with scratcher electrodes, gamma-ray log, neutron log, section-gauge log, 

 and temperature log. All these methods can be applied for the definition of 

 beds and correlation. For reservoir analysis the neutron log provides formation 

 factor and porosity determination, and the induction log gives a record of Rf. 



Interpretation of Electric Logs 



Standard Technique 



As seen under Fundamentals of Electric Logging, the water saturation 

 is S w = t/ R /R.£. If we assume that the true resistivity (R t ) can be obtained 

 from the bed under study, it suffices to get the value of R corresponding to 

 the same bed. Such value of R can be obtained at the bottom of the section 

 under study or in the same sand in a nearby well. In this instance, the water- 

 sand resistivity can be used effectively as long as both the porosity and the 

 salinity of the formation water are constant throughout the section. 



It has also been shown that the resistivity of a water sand is equal to the 

 product of the formation factor F and the formation-water resistivity K w : 

 R = FR W . It is also shown that the formation-resistivity factor is related to 

 porosity values by means of comparatively simple empirical relations. 



It is always easy to calculate the value of R corresponding to a given 

 formation when one knows the formation factor, F, (or porosity, <j>) and the 

 water resistivity, K w . The formation factor can often be obtained with sufficient 

 accuracy with the MicroLog and the MicroLaterolog. The water resistivity 

 can be derived from the SP curve. 



Rocky Mountain Method 



In formations when R16" > 10 R„ t (R16" is given by the small normal of 

 16" spacing), a method was found to give good interpretation when the small 

 normal reading, the long lateral, and the SP were used. This method is known 

 as the Rocky Mountain Method because it was originated in that region. 



From the small normal curve, a value for R i? the invaded-zone resistivity, 

 is obtained by correcting the small normal for hole effect. 



320 



