MAGNETIC METHODS 221 



the magnetic susceptibility of cores, if available, for a well in the area. If 

 highly magnetic sedimentary beds were indicated and the basement was 

 considered to be at great depth, a sedimentary picture would be suggested. 

 A third means, though still less reliable, would be the application of pole 

 depth rules to the magnetic anomaly, to determine whether the depth to the 

 pole would indicate it lay in the basement or in the sedimentary section 

 above. A combination of magnetic and gravity measurements may give a 

 clue to the probable depth of the basement rock. 



An example of a magnetic low associated with a known structural high 

 is furnished by the Garber Oil Field, in Garfield County, Oklahoma. The 

 Garber Field lies on the southern extension of the Nemaha granite ridge, 

 which runs approximately north-south across the center of the state of 

 Kansas and into Oklahoma. Drilling at Garber indicates a relatively thin 

 section of the Arbuckle limestone (1,250 feet) on top of the pre-Cambrian 

 granite of the ridge. The Arbuckle is practically non-magnetic, and has 

 been lifted about 2000 feet in this anticline. In this general area it is 

 normally overlain by up to 6000 feet of highly ferruginous shales and 

 sandstones. 



The magnetic low shown at the Garber field could be explained on the 

 assumption that the granite, which is relatively non-magnetic, protrudes 

 through strongly magnetic sediments and possibly some schists, which have 

 thus been cut out.f 



The Hobbs Oil Field, of Lea County in southeastern New Mexico, is a 

 case in which there was fair agreement between a magnetic high and a 

 structural high. (See Figure 105.) 



The field was originally discovered as the result of magnetic work in 

 1926, and checked by a traverse of torsion balance stations. The discovery 

 well (1928) was located in the center of the magnetic high, which fortun- 

 ately was sufficiently "on structure" to give a small producer. | Figure 106 

 shows the magnetic contours and the final structure contours based on well 

 logs obtained as the field was developed. § In this case there is a shift of 

 about 8,000 feet between the center of the structural high and the center of 

 the magnetic high produced presumably by the basement rock. The field 

 includes approximately 6,000 acres and 143 producing wells. Two oil pro- 

 ducing zones exist: the Bowers sand at a depth of 3,170 to 3,225 feet; and 

 the white lime, a white or crystalline lime of Permian age occurring at a 

 depth of about 4,000 to 4,200 feet. The sedimentary series has a magnetic 

 permeability of approximately one, and the basement granite has a relatively 

 high magnetic permeability. The topography is flat, and the surface has a 

 southeast slope of about 10 feet to the mile. The surface formation is 

 caliche, and there are no outcrops. 



t D. Wantland, "Magnetic Interpretation," Geophysics, Vol. 9, No. 1, Jan., 1944, pp. 47-59. 



t C. B. Carpenter and H. B. Hill, "Petroleum Engineering Report, Big Spring and Other Fields 

 in West Texas and Southeastern New Mexico," Dept. of Interior, R.I. 3316, Nov., 1936. 



§ A somewhat similar figure is shown in a pamphlet entitled "Mapping Geological Structure with 

 the Magnetometric Methods," published by William M. Barrett, Inc., Shreveport, Louisiana. 



