CHAPTER 1 

 INTRODUCTION 



L. W. LeROY 



Subsurface geology involves interpretation of the stratigraphic, struc- 

 tural, and economic values below the earth's surface. These interpreta- 

 tions are based on information obtained from bore holes, geophysical 

 data, and projected surface information. As stated by Jakosky,^ "The sub- 

 surface is an extremely complex three-dimensional region, which the in- 

 terpreter must diagnose with only the help of the very limited type of 

 data obtained from our present techniques." 



The subsurface geologist is required to have: (1) a basic back- 

 ground in geologic structure, (2) a broad and fundamental knowl- 

 edge of rock types, (3) a three-dimensional concept of geologic phe- 

 nomena, and (4) an understanding of the economics of the problem. He 

 must be able to coordinate and accurately integrate these related phases. 

 He must realize the importance of structure to stratigraphy and strati- 

 graphy to structure. He must be aware that stratigraphic and structural 

 problems cannot be solved without first evaluating the rocks and their 

 relationships. To place rock types in their proper categories, emphasis 

 must be made on the various techniques and methods which permit more 

 exacting classification. Electrical, radioactive, caliper and other logging 

 data cannot be accurately interpreted until the lithologic aspects of the 

 strata have been established. The rocks and structures developed within 

 them must be treated from the three-dimensional viewpoint because both 

 change in space. Folding intensities and characteristics and lithologic 

 patterns exhibit variations which must be recognized before appraising 

 the geologic impress. Subsurface geology demands a creative imagination, 

 an analytical and systematic approach, and a multiple-hypotheses manner 

 of thinking. The geology of tomorrow depends largely on the assertive- 

 ness of the subsurface geologist of today. 



Subsurface geology as applied in the petroleum industry has made 

 rapid advances since 1925. In many areas it has attained greater impor- 

 tance than surface geology; the discovery of most future oil fields of 

 the world will undoubtedly be attributed to subsurface geologic studies. 

 Oil operators today fully realize the exigency of appraising subsurface 

 conditions in exploration and development programs. 



Methods applied in evaluating subsurface conditions vary in nature 

 and complexity, depending on the character of the rocks penetrated, the 

 type of equipment available, the quality and quantity of data desired, and 

 the time allocated to the solution of the problem. 



^ Jakosky, J. J., Whither Exploration: Am. Assoc. Petroleum Geologists Bull., vol. 31, no. 7, 

 p. 1121, July 1947. 



