Chap. 11] GEOPHYSICAL WELL TESTING 831 



Of considerable practical interest is the problem of how the trite forma- 

 tion resistivity changes with the proportion of salt water and oil. Such 

 experiments have been conducted by Martin, Murray, and Gillingham"' 

 (see Fig. 11-4). Oil contents up to 60 per cent increase the formation 

 resistivity about ten times. As was stated before, (see page 665), this ratio 

 between the resistivities of productive and barren formations is frequently 

 encountered in the practice of electrical logging. It is further seen in 

 Fig. 11-4 that oil contents exceeding 70 per cent bring about a sharp in- 

 crease in formation resistivity, of the order of one hundred to several 

 thousand times. 



B. Determination of Spontaneous Potentials (Porosities) 



When porous formations are penetrated by the drill, they give rise to 

 spontaneous potentials in two ways: (1) by movement of liquids through 

 the formation into or from the hole (electrofiltration potential), and (2) by 

 differences in concentration between formation water and drilling fluid 

 (diffusion potential). The former, discussed on page 631, is practically the 

 only source of spontaneous potentials in the absence of concentration 

 differences between formation water and drilling fluid. These potentials 

 are positive when water is discharged into the hole but negative when 

 water flows from the hole into the formation. Hence, the magnitude of 

 the potential anomaly depends on the pressure or height of the mud 

 column in the hole. The extreme potential anomaly in the well illustrated 

 in Fig. 11-5 was ±20 millivolts. No diffusion potential was present, since 

 the mud contained about 2 g NaCl per liter and the formation water about 

 3 g NaCl per liter. Usually the electrofiltration potentials are negative 

 in sign, since the drilling fluid penetrates the porous formation under 

 excess pressure. 



Diffusion potentials are produced by porous formations because the 

 (fresh water) drilling fluid is generally lower in ion concentration than is 

 the (connate) formation water. As shown in Fig. 1 1-6, currents flow from 

 the drill hole into the layer and cause negative potential peaks which may 

 amount to 100 to 200 millivolts. This phenomenon was discussed on 

 page 631. The theoretical potential difference, according to eq. (10-2), is 

 E = 11.6 logio P2/P1 millivolts, so that, for a formation water of 0.5 ohm-m 

 and a drilling fluid of 5 ohm-m resistivity, the potential difference would 

 be 11.6 millivolts. However, because of an additional potential difference 

 set up along the return circuits at the points CB and C'B', respectively, 

 the potential difference is actually greater than for only one boundary, so 



Geophysics, 111(3), 258-272 (1938). 



