GEOPHYSICAL METHODS IN OIL 

 EXPLORATION 



The following chapter is intended to assist the operator in the selection 

 of the correct geophysical method when prospective oil territory is to be 

 surveyed. It contains a discussion of (1) the general possibilities of 

 geophysical methods, with chief reference to new areas where the expected 

 type structure is unknown; (2) the choice of methods for specific types of 

 structures; and (3) other nongeological considerations in the selection of 

 methods. 



If nothing whatever is known about the geology of an area, the most 

 expedient procedure is to make a magnetic reconnaissance survey with 

 widely spaced stations. This may be expected to indicate the topography 

 of areas of the basement rocks, provided they are uniformly magnetized. 

 After promising high areas have been located, the magnetic discoveries 

 should be followed up (preferably by seismic-reflection methods) to deter- 

 mine whether the basement uplifts are accompanied by structure in the 

 overlying sediments. In certain areas magnetic anomalies may also be 

 caused by magnetic sediments. Cases have been known where salt domes 

 have furnished magnetic indications, and where anticlines and faults could 

 be traced by virtue of the presence of magnetic beds in the stratigraphic 

 sections. If the basement rocks are not uniformly magnetized, a magnetic 

 high may simply mean an area of greater magnetization or an area occupied 

 by more magnetic rocks. In some instances it has been found that the 

 magnetization was the reverse of that normally expected and that magnetic 

 highs indicated structural lows. 



Where magnetic anomalies have no structural significance, it is necessary 

 to use other reconnaissance methods — for instance, the gravitational 

 method. A survey made with pendulums or gravimeters can generally be 

 depended upon to depict fairly accurately the major structural trends. 

 In a few instances it has been found that gravity highs do not reflect 

 geologic structure in the oil-bearing strata. In such cases the work has to 

 be conducted by methods, such as seismic refraction or reflection, which 

 rely only on the structure in the sedimentaries. 



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