40 EXPLORATION GEOPHYSICS 



Exploration techniques have followed cycles, each of which covered a 

 period of relatively few years. The first technique utilized surface indica- 

 tions such as oil seeps. Then came the period of creekology, which ushered 

 in the trend theory; then the reversion to surface indications, as geology 

 and plane-table mapping were employed to find structural highs. During 

 this time much wildcat drilling was b^ing done. The wells were not too 

 expensive due to their shallow depth. 



Systematic core drilling techniques in slim-holes were introduced to 

 work out structure covered by shallow overburden and unconformities. 

 The relatively high cost of such exploratory drilling and the increased cost 

 of deeper wildcat wells encouraged the use of geophysical methods to map 

 the subsurface structure. Then came the peak periods and cycles of appli- 

 cation of refraction seismic, torsion balance, magnetometer, reflection 

 seismic, gravity meter, and some geochemical and electrical work, in the 

 order listed. Slim-hole drilling with electrical logging was also developed. 



Present Exploration Trends. — Today, for lack of better methods, as 

 we continue our search for the subsurface structural highs, the trend in 

 general is toward the ultra-refinement of present techniques, which com- 

 prises the careful re-evaluation of an area. This requires the use of more 

 intensive geology, where possible, supplemented by two or more geo- 

 physical mthods. By correlation of the data from different geophysical 

 methods, it often is possible to interpret some of the weaker anomalies, 

 the significance of which is not apparent from the application of one single 

 geophysical method. Obviously the cost of such work, necessitating the 

 combination of independent techniques, must be high. 



If it were not for the limitations imposed by economic considerations, 

 it would be possible to continue along the present lines of endeavor with 

 more and more intensive application of the combined geophysical and geo- 

 logical procedures. Such a program will of course find additional oil, but 

 its use can result only in increasingly greater costs. Thus the law of dimin- 

 ishing returns enters the picture. Often the development of deeper 

 structures is not overly attractive due to the greater cost of deep drilling, 

 coupled with the fact that the deeper horizons generally have a smaller 

 recovery per acre. The increased development and lease operating costs, 

 together with the lower productivity, far too often make the exploitation 

 of a deep-lying structure a risky marginal operation. 



These conditions will probably furnish an impetus to the search for 

 relatively shallow occurrences in new areas and for shallow stratigraphic 

 traps and low closure structures in or adjacent to present producing areas. 

 In the United States, the most important undeveloped regions remaining 

 today are the covered or masked areas, complex fault accumulations, and 

 the stratigraphic traps created by unconformities, pinch-outs, changes in 

 porosity or sedimentation, and lensing. Nearly a third of the important oil 

 fields in this country are of the stratigraphic type, and were discovered by 



