1110 Subsurface Geologic Methods 



no specific reflecting beds. A dead zone of no reflections may be seen on 

 the profile (fig. 587) at depths between 1,200 and 1,650 meters. This is 

 identified as the Freites-shale section, which is approximately 500 meters 

 thick in this area. For 500 meters below the Freites section the alternating 

 sands and shales of the Oficina and Temblador formations afford sufficient 

 contrast to reflect seismic energy. These beds rest upon the basement 

 rocks. The fault shown in the profile closed the contours on the upthrown 

 side sufficiently to form an oil reservoir.^* 



Loudon Oil Field, Fayette and Effingham Counties, Illinois — In order 

 to evaluate a large block of acreage on the west flank of the Illinois Basin, 

 a reconnaissance jump-correlation survey was made in the Loudon field 

 of Illinois by the Carter Oil Company in 1936. Using shot points spaced 

 one mile apart, the entire closure was mapped in less than a month. The 

 results of this survey are shown in figure 588. On the results of the 

 correlation survey additional acreage was obtained. The completed block 

 of leases embraced a large part of the ultimate producing area. Several 

 continuous lines were run across the area to check the correlation results; 

 however, only slight modification of the contour pattern was required. 

 The discovery well was located on the seismic high. The field has 1,940 

 producing wells, and the ultimate recovery is estimated at 193,500,000 

 barrels. Close agreement may be seen between the structural contours on 

 the Devonian and the seismic contours on the Trenton formation 900 feet 

 below.^^ 



Electrical Prospecting 



The electrical methods of geophysical exploration make use of several 

 types of electric fields; therefore, the methods of observation are several 

 and varied both as to technique and the properties measured. Certain of 

 the methods utilize natural electrical currents that flow through the earth. 

 Others use controlled currents obtained from batteries or mobile gen- 

 erators that are introduced into the ground by either direct contact or 

 by induction. The operator has the choice of introducing into the ground 

 either direct current or alternating currents of various frequencies. Only 

 a few of the many electrical methods that have been proposed for ex- 

 ploration will be discussed here. 



Application of electrical methods has been made chiefly in the mining 

 industry, owing to the comparatively shallow range of investigation in- 

 herent in the methods. Exception must be made to the wide-spread use by 

 the oil industry of the electric-well-logging technique. Recently, interest 

 has been shown by a number of oil operators in the telluric-currents 

 method of prospecting, which promises to become an important recon- 

 naissance tool. 



^ Olson, W. S., op. cit., pp. 611-618. 



^ Lyons, P. L., Geophysical Case Histories, vol. 1, pp. 461-470, Tulsa, Soc. Exploration Geo- 

 physicists, 1948. 



