564 busch. SUBSURFACE TECHNIQUES [Ch. 31 



water is generally fresh, any sand is likely to have a high resistivity 

 near the borehole. If the three electrodes in the hole are closely 

 spaced, the resistivity might be measured within a zone smaller than 

 the depth of penetration of water filtrate from the drilling mud. The 

 resultant resistivity curve would be that of the drilling fluid plus 

 any dilution effects of the connate water of the surrounding strata. 

 This effect is partially corrected by making additional resistivity runs 

 with the suspended electrodes more widely spaced. In this way the 

 radius of investigation of the electrical field is greater than the depth 

 of penetration of the drilling fluid. 



The resistivity curves obtained by additional runs commonly are 

 referred to as third and fourth curves. These curves are measures of 

 resistivity of larger intervals of rock than the second curve, and, al- 

 though they more accurately portray the electrolytic character of the 

 original fluids in the formation, they are less precise indicators of the 

 tops and bottoms of the various strata. Three resistivity curves (sec- 

 ond, third, and fourth curves) are illustrated on the electric log of 

 Fig. 1. It is more customary, however, to use only two such curves, 

 as shown on the electric log of Fig. 2. 



In general, shales have fairly low resistivity because of the salt 

 water contained in their pores. Salt-water sands of high porosity have 

 still lower resistivity. Oil and gas sands generally have higher re- 

 sistivity than shales, depending on the amount of connate water they 

 contain. Limestones generally have high resistivity. Very dense 

 sands also have high resistivity, as do fresh-water sands. 



It was believed at first that the resistivity log would indicate whether 

 the permeable beds contained oil or water. Unfortunately they do not. 

 The true resistivity of the rock cannot be determined because of the 

 mud-filtrate invasion, and because the presence of the borehole and 

 the finite thickness of the beds introduce variable correction factors 

 that can seldom be evaluated. Some oil sands contain so much con- 

 nate water that they show low resistivity. 



Accordingly, electric logs cannot be used to estimate either per- 

 meability, porosity, or fluid content of subsurface formations. How- 

 ever, changes in lithologic properties of rocks are nearly always ac- 

 companied by changes in electrical properties. Thus all the major, and 

 many of the minor, lithologic variations in the rocks exposed in a bore- 

 hole are shown as shifts in the electric-log curves. It is usually easy 

 to distinguish sand from shale, even though not much can be told about 

 the physical properties of either. Limestones are generally also recog- 

 nizable. The depths to the lithologic changes are determined to within 



