560 busch. SUBSURFACE TECHNIQUES [Ch. 31 



anticlinal structures, give clear-cut evidence that something more 

 fundamental than closures governs the distribution of oil and gas in 

 these pools. That "something" is sedimentation and stratigraphy. 



A sedimentological approach to the problem of locating oil is not 

 predicated on any one factor as a cause of accumulation, and usually 

 it involves numerous techniques. In the past few years the discovery 

 of oil in reefs in such pools as the Leduc (Link, 1949, pp. 381-402) and 

 Redbank of Alberta and the Marine pool (Lowenstam, 1948, pp. 153- 

 188) of Illinois has served to focus the attention of the oil man on the 

 combined techniques of the structural and the sedimentological geolo- 

 gist. The geophysicist has refined his techniques so as to locate reefs 

 that are overlain by arched strata. Where there is little or no reflec- 

 tion of structure above a reef, detailed paleogeographic and facies 

 studies hold out the greatest promise for adequate techniques in finding 

 them in the subsurface. 



PRINCIPAL SOURCES OF SUBSURFACE DATA 



Well logs are the most important single tool in subsurface strati- 

 graphic analysis. When they are used in conjunction with other types 

 of data, a fairly complete picture can be reconstructed of the rock 

 strata far below the surface of the earth, that is, the types of lithology, 

 their respective thicknesses, structural configuration, and facies rela- 

 tionships. Well logs are of several types, but all have one feature in 

 common in that they are all either direct or indirect records of the 

 various types of strata penetrated by the bit in the drilling of the hole. 



The most prevalent types of logs currently in use are illustrated 

 in Fig. 1. All these logs were made from data on the same borehole, 

 and they serve to illustrate the precise nature of the information avail- 

 able to the geologist interested in making subsurface stratigraphic 

 studies. Frequent reference to Fig. 1 will prove useful to a better 

 understanding of the following discussion of different types of logs. 



SAMPLE LOGS 



In cable-tool drilling, samples may be bailed directly from the 

 bottom of the hole every time an additional 5 to 10 feet of new hole 

 has been made. When drilling in the producing horizon the bailer 

 may be run every foot or two. In all events the rock cuttings dumped 

 from the bailer are quite representative of the formation drilled since 

 the previous run of the bailer. In rotary drilling it is impossible to 

 obtain samples as good as those obtained in holes drilled with cable 



