GEOLOGIC AND ECONOMIC BACKGROUND 31 



measurements are made in the drilled well, or while drilling, and are plotted 

 as a continuous log. From a study and comparison of the logs of wells in 

 an area it is generally possible: (1) to differentiate between beds of shale 

 and sand, (2) to distinguish sands containing oil or gas from those carry- 

 ing salt and fresh water, and (3) to make stratigraphic correlations and 

 thus determine structure. These methods are discussed in detail in 

 Chapter XI, 



Seismic Methods 



The seismic methods have proved generally the most applicable and 

 uniformly successful of the various geophysical methods used in oil explor- 

 ation. Two general types of seismic technique are employed: (1) refrac- 

 tion surveying and (2) reflection surveying. 



The refraction seismic method has been used most extensively and suc- 

 cessfully in the search for shallow salt domes in the Gulf Coast of Texas 

 and Louisiana. The characteristic higher velocity of the salt core of the 

 domes renders this type of structure readily amenable to refraction tech- 

 nique. In some regards, the general limitations of refraction technique 

 are similar to those of electrical methods, particularly in that increasing 

 depth results in rapidly increasing costs and loss in detectability. 



A somewhat different form of refraction seismic work was developed 

 shortly before World War II. It is characterized by exceptionally long 

 distances (up to several miles) between the shot points and the geophones. 

 Quite often two shooting trucks are used. It has been found that the depth 

 to a refracting horizon, or refractor, can be determined along exploration 

 lines at as many points as desired. Such a refractor (often more than one) 

 can be mapped and correlations of its geologic position in the section estab- 

 lished from drilled wells. This work can be carried to a depth of more 

 than 12,000 feet without difficulty. The technique has been successfully 

 used for mapping deep subsurface structure in the Edwards Plateau area 

 of west Texas, in the Anadarko Basin north of the Amarillo buried granite 

 ridge, in the West Texas Permian Basin, and elsewhere. It is somewhat 

 more costly than the shallow refraction work mentioned above. 



Reflection seismic surveys are currently the most popular and success- 

 ful of the geophysical techniques used in exploration for oil structures. 

 They have been employed in areas containing practically all types of geo- 

 logical structures important in oil exploration, and in most of the major 

 oil-producing provinces of the w^orld. They are of special value in mapping 

 geological structure in sedimentary beds to depths of 15,000 feet. This is 

 usually accomplished by one of three techniques : (a) by correlating corre- 

 sponding reflections from the same formation over an area, (b) by com- 

 puting isolated dips (dip-shooting) of the formations from which reflec- 

 tions are obtained, or (c) by continuous correlation of short interlocking 

 profiles obtained from closely spaced shot points. The procedure is called 

 continuous profiling. 



