BORE-HOLE INVESTIGATIONS 1017 



ELECTRICAL METHODS 



In addition to the location of subsurface geological markers which 

 are of value for correlation purposes, the proper combination of resistivity 

 and potential measurements vi^ill oftentimes show the presence of oil sands, 

 with particular reference to the boundary between oil and water-bearing 

 formations. Although the electrical resistance of oil is very high, that of 

 the formations which contain oil is not correspondingly high, due largely to 

 the presence of connate waters in the formations. This factor, and its effect 

 on electrical resistance, has been discussed in connection with Figure 264. 

 However, the oil-bearing formations generally do have a measurably higher 

 resistance than the water-bearing formations which are usually adjacent to 

 them. Hence, a relatively high electrical resistance may be indicative of 

 an oil-bearing formation. 



In certain oil fields, the parameters suppHed by resistivity and potential 

 measurements are quantitative and depend on the amount of oil in the 

 formation ; hence, such measurements occasionally are useful for ascertain- 

 ing the general productivity.! However, due to the fact that there is no 

 direct relation between the quantities measured (resistivity and earth poten- 

 tials) and the information desired (production in barrels per day), infer- 

 ences concerning productivity are entirely empirical and are valid only for 

 the particular field and sand for which the empirical relationships were 

 established.* 



In mining work, drill-hole measurements are often useful for determin- 

 ing the proximity of ore bodies and the extent of mineralization. In this 

 application, electrical drill-hole exploration may be considered as a means 

 of extending the diameter of the drill hole. The history of mining ex- 

 ploration is replete with cases where commercial ore bodies have been 

 missed by very short distances by drill holes. A contributing factor to this 

 condition is the crookedness or drifting of drill holes. 



The electrical records are similar to the common core and sample 

 records, and they are, for that reason, termed "electrical logs." These logs 

 generally may be obtained without an appreciable increase in drilling time 

 and at a small fraction of the cost of mechanical coring. Usually, a com- 

 plete and continuous electrical log of a drill hole may be obtained at a cost 

 equivalent to only a few feet of mechanical coring. 



t M. Martin, G. H. Murray, and W. J. Gillingham, "Determination of the Potential Productivity 

 of Oil Bearing Formations by Resistivity Measurements," Geophysics, July 1938, Vol. 3, No. 

 3, pp. 258-271. 



*Production is often affected by conditions created during the drilling operations, 

 for instance, the deposition of a mud sheath on the wall of the well opposite the oil- 

 and gas-bearing formation. (Compare C. P. Bowie, "Hardening of Mud Sheaths in 

 Contact with Oil, and a Suggested Method for Minimizing their Sealing Effect in 

 Oil Wells," Bureau of Mines Report of Investigation No. 3354.) 



