28 EXPLORATION GEOPHYSICS 



producing oil fields are masked by soil, alluvium, glacial deposits, sand 

 dunes, or water which, to date, have prevented or seriously impeded the 

 determination of deeper structure. Regional geological considerations of 

 such areas may indicate the presence of probable petroleum source beds, and 

 the possible existence of favorable traps for oil, below the concealing mantle 

 of surface materials. Notable examples of such areas in the United States 

 are the glacial drift-covered parts of Michigan, Indiana, Illinois, Missouri, 

 Nebraska and Iowa, and the extensive alluvium-covered San Joaquin and 

 Sacramento valleys in California.f In such regions, the possibility of addi- 

 tional future discoveries of petroleum rests largely on the application of 

 improved techniques in geophysical exploration. Even in cases where struc- 

 ture of the surface formations can be mapped satisfactorily, the projection 

 of exposed structure to the depths at which oil accumulation might be 

 expected often introduces a high element of risk because of obvious struc- 

 tural asymmetry, or the possible occurrence of unconformities or appre- 

 ciable variations in the thickness of subsurface beds. 



Many known accumulations of petroleum occur in sand lenses and other 

 types of stratigraphic traps not necessarily associated with closed anticlinal 

 structures. The discovery of such stratigraphic traps is a difficult problem 

 and not much help can be obtained from our current (1949) geophysical 

 technique. Present methods of approach to this problem include : the appli- 

 cation of such knowledge of the regional and local geologic history as is 

 available ; study of subsurface structure as determined by geophysical work 

 or previously drilled wells in the area ; slim-hole drilling ; and geochemical 

 surveys. 



Geophysical exploration for petroleum structures consists primarily in 

 locating and delineating those types of subsurface structure which experi- 

 ence has shown to be favorable for oil accumulation. The success of the 

 methods to date may be measured, therefore, by the extent to which they 

 have disclosed favorable subsurface structures for which geologic evidence 

 was previously uncertain or lacking. By this criterion and the many prolific 

 new fields so discovered, petroleum geophysics has become established as a 

 necessity in most exploration programs. 



Relative Utility and Importance of the Methods 



Magnetic Methods 



Magnetic methods have been widely employed in some areas for specific 

 problems in oil exploration. Their greatest success has been in certain cases 

 of uniquely favorable geological conditions wherein strong magnetic ano- 

 malies are directly associated with the oil-bearing structure. For instance, 

 magnetic methods have been used successfully to map the trends of buried 

 igneous ridges and other features of basement topography, which from 

 previous experience are known to have, commonly, a definite relation to 



t W. G. Osborn, (1) "Geological Complex of Iowa," The Oil and Gas Journal, May 13, 1937; 

 (2) "Geologic Aspects of the Forest City P.asin," The Oil and Gas Journal, January 5, 1939. 



