ent companies and different geologists, each with a different background of train- 

 ing and experience, will attack the exploration of a region differently, and some 

 phase may be omitted. If there are no outcrops, for example, surface mapping 

 may be omitted, and geophysical mapping may begin at once. If a purely strati- 

 graphic trap-pool is discovered early, more attention will be given to strati- 

 graphic exploration in the early stages. On the whole, however, the more ex- 

 ploration there has been, the higher will be the proportion of effort devoted to 

 subsurface mapping, both structural and stratigraphic. 



SUBSURFACE GEOLOGY Subsurface geology increases in usefulness as 



more drilling is done in a region principally 

 because (1) the more exploration there has been the more difficult it is to find 

 new structural features to drill, and more attention is given to stratigraphic 

 anomalies that can be determined only by subsurface mapping, and (2) the 

 more exploration, the more data there are available for study and comparison 

 with past data. 



The emphasis in subsurface geology changes with time. As the exploration 

 of any region develops, more attention is given to subsurface methods for map- 

 ping structure than is given to surface and geophysical techniques. Once a struc- 

 ture is located and mapped, there is seldom a need for complete remapping. 

 The emphasis then shifts to any of a variety of stratigraphic features associated 

 with the structure that might form traps, and these require a revaluation as every 

 new log in the vicinity becomes available. Finally, an accurate understanding 

 of the geologic history of a region calls for mapping every imaginable phe- 

 nomena and episode that can be located until the history is unravelled. 



There is also a need for subsurface geology during the development of a 

 pool. The subsurface geologist is accustomed to working with all kinds of well 

 logs and well data, and his interpretations and thinking are essential to the petro- 

 leum engineer's understanding of the geological conditions associated with the 

 reservoir. After all, much of the development and production of a pool are but 

 extensions of principles that the geologist uses in his daily thinking during the 

 search for a pool. 



The well log is the basic source of information in subsurface mapping. Sig- 

 nificant and useful facts are continually being squeezed out of the old logs, and 

 development of new ways of logging are almost an annual occurrence. No area 

 has been mapped completely until the most modern logging methods have been 

 applied to it. Even where the logs are old, one modern log may help considerably 

 in deciphering the old log. The sample log, the paleontologic log, the time log, 

 the electric log, and the continuous-velocity log add new data to the stratigraphic 

 record. These data, after being evaluated, can be applied frequently to the drillers' 

 logs to make them more useful than before. 



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