36 EXPLORATION GEOPHYSICS 



note that the usual type of stratigraphic trap is associated with homoclinal 

 structure.) If such a structure is sealed by a fault or by an igneous dike, 

 or other intrusive, the location of possible oil accumulations may be 

 determined indirectly by locating the impounding fault, dike, or intrusive. 

 Magnetic, electrical, gravitational, and seismic methods have been employed 

 in fault location. However, direct location of faults in sedimentary sections 

 generally is most economically accomplished by use of electrical methods. 

 If the impounding structure is an igneous dike or other type of igneous 

 intrusion, the magnetic, and in some cases the gravitational, methods are 

 clearly the most applicable. 



As indicated previously, the application of geophysical methods to the 

 solution of the general problem of locating stratigraphic traps has not 

 been very successful. Oil accumulation may occur in the up-dip edges of 

 sand lenses and beds truncated by an unconformity on broad structural 

 arches or plunging anticlinal noses. The reflection seismograph generally 

 can reveal the location and depth of these features, but, except in cases of 

 large divergence, can seldom provide an indication of the occurrence and 

 location of the stratigraphic trap itself. Slim-hole drilling, although com- 

 monly expensive, offers the most positive method of exploring for such 

 traps. Geochemistry or soil analysis may ultimately provide a direct 

 method of locating oil-containing traps of this type. Stratigraphic traps 

 are responsible for the East Texas field, the East Coalinga field, much of 

 the accumulation in the Midway- Sunset, and many other important pro- 

 ducing areas scattered throughout the United States. Accumulations of oil 

 in traps frequently are extensive and profitably recoverable. The future of 

 petroleum exploration depends to a large extent upon the development of 

 methods to locate the stratigraphic traps that still remain hidden. 



In predicting the future tasks of exploration geophysics, it may be 

 assumed that the oil traps still to be found are similar to those known to 

 contain accumulations. Hence, if present methods and future improve- 

 ments in geophysical techniques are to be successful in the search for yet 

 undiscovered accumulations of oil, they must be adapted to locating: (1) 

 anticlinal structures of low relief, (2) structural closures against faults, 

 and (3) stratigraphic traps, whether they be depositional sand lenses, 

 unconformities, or lenticular zones of porosity resulting from chemical 

 action. 



Economics of Petroleum Geophysics. — Emphasis in the preceding 

 sections has been placed mainly upon the geological considerations influ- 

 encing choice of methods, with only brief reference to cost. Cost of geo- 

 physical surveys is mainly determined by : ( 1 ) equipment, the necessary 

 operating personnel, and supplies, (2) quantity and quality of information 

 required, and (3) terrain and other conditions affecting speed of operation. 

 The amount of detail required is obviously a function of the amount of 

 geological information already available, the complexity of the problem, and 

 the general purpose of the survey. Speed of operation and coverage is 



