All analyses of stratigraphic methods in exploration point to the increasing 

 need of greater precision. The direction is toward better logs; more accurate 

 correlations, contacts, facies changes, unconformities, and truncations; to more 

 data on porosities and permeabilities; and to better fluid and reservoir charac- 

 teristics such as saturations, pressures, and temperatures. 



The relationships between structural and stratigraphic phenomena in the 

 formation of a trap are shown in Figure 2-1. The gradation from 100 percent 

 structural causes to 100 percent stratigraphic causes is shown diagramatically. 



THE SUBSURFACE The first objective of the petroleum geologist 



GEOLOGIST is to discover oil and gas; this is the chief 



reason for his existence. As long, then, as 

 there is a need for more petroleum discoveries, there will be a need for the petro- 

 leum geologist — first for his ability to work geology at the surface, then, and 

 probably more important, his ability to work geology below the surface. 



Subsurface geology, like other kinds of geology, is dynamic. New ways are 

 continually being found for obtaining subsurface information, or interpreting 

 data, and for predicting the position of a favorable prospect. The subsurface 

 geologist must, therefore, be alert to change and be ready to use new kinds of 

 information, to re-examine his old data for new meanings, and to put his findings 

 together in new ways. Not only are new techniques continually being discovered 

 but, once discovered and found to be practical, there is a steady advance in their 

 interpretation and in the construction and operation of the equipment. 



Geophysical surveys are an integral part of any modern exploration pro- 

 gram. A geophysical survey is, in fact, a subsurface geological survey conducted 

 from the surface without drilling. The records might be thought of as logs of the 

 rocks below the surface, and they require geological interpretation exactly as 

 do well logs. It is essential that any geologist working on subsurface problems 

 should be familiar with the advantages and the limitations of the different geo- 

 physical methods of surveying. 



Geophysical data should be fitted into other subsurface data and all of the 

 information synthesized into a complete picture. This procedure becomes more 

 and more important as the structural closure diminishes, for less and less relia- 

 bility can be placed on minor structural features, especially where the measure- 

 ments are near or below the limits of error of the instruments, and more and 

 more reliance must be placed on stratigraphic anomalies. Geophysical surveys 

 of all kinds are steadily improving in accuracy, however, and areas that could 

 not be precisely mapped even in the recent past may suddenly become mappable. 



One corollary of an increase in subsurface mapping is that it requires more 

 geological imagination than surface or geophysical methods. The reason is that 

 the data are frequently widely scattered, insufficient, or inconclusive; and unless 



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