16 EXPLORATION GEOPHYSICS 



progress in Oklahoma, Kansas, and some parts of the Gulf Coast, when it 

 was abruptly superseded by the more rapid and economical reflection seis- 

 mic method, introduced in 1927. However, during the past few years 

 increasing use has been made of another type of drilling method, termed 

 "slim-hole" drilling. 



Direct exploratory methods, such as drilling, are a means of obtaining 

 the most definite and irrefutable type of data. The information from such 

 a direct method possesses three components that are most desirable : ( 1 ) 

 direct determination of the structural conditions existing at a specific point 

 or localized area, as contrasted to geophysical methods which involve 

 measurements over areas of relatively large extent;* (2) specific informa- 

 tion regarding the strata traversed by the bore hole, by means of cores or 

 electrical logs; and (3) accurate depth measurements. 



In present commercial usage, slim-hole drilling constitutes the drilling 

 and electrical logging of a small diameter hole. Successful work of this type 

 is being conducted to depths as great as 3,000 - 5,000 feet. The subsurface 

 structure is determined by the correlation of the electrical logs from holes 

 properly spaced in the area under investigation. Any desired portion of a 

 hole may be cored to obtain samples for lithologic, paleontological, permea- 

 bility and porosity studies and to determine the presence or absence of oil. 

 Substitution of an electrical log for the complete core efifects a saving which 

 makes the method economically feasible in many areas, especially the Gulf 

 Coast, where soft strata permit rapid drilling at low cost. There is a possi- 

 bility of obtaining greater advantages and of effecting still further economy 

 by the use of some form of electrical or mud-logging technique while 

 drilling is in progress. Present commercial practice indicates that a 4^'' 

 hole is the minimum diameter if the drilling operation is to be followed by 

 electrical logging. If a usable log could be obtained while drilling, the 

 diameter of the hole could be reduced, with a material saving in drilling 

 time and cost. Work now under way indicates that the logging-while- 

 drilling techniques may be able to utilize holes of about 2 to 2J/2 inches in 

 diameter. The limiting minimum diameter appears to be governed by the 

 ability to circulate sufficient water or mud to clear the hole of cuttings. 



Slim-hole drilling, with coring at desired intervals, may be combined 

 with core analysis and orientation to produce an exploration procedure far 

 surpassing in accuracy any technique in use today. If greater economy can 

 be achieved in the drilling and logging technique, the cost of such work 

 may become commercially feasible for general exploration. Considerable 

 experimental work has already been done using slim-hole drilling as con- 

 trol, with holes spaced one to five miles apart, and other low-cost geophys- 

 ical methods for structural mapping between holes. Successful develop- 



* Reflection seismic work has a relatively good resolving power because its zone 

 of measurement is small as compared to gravitational, magnetic, electrical, and other 

 potential methods, which are decidedly "weighted" types of measurements involving 

 zone of measurement of somewhat indefinite lateral extent and depth. 



