Introduction 7 



and shown in map form. All such maps permit an improved 

 understanding of subsurface conditions. 



8. Unconformities : The character, extent of erosional surfaces, and 

 the relationship of such surfaces to adjacent strata are often 

 much improved by subsurface information. These surfaces 

 must be accurately defined before stratigraphic and economic 

 values can be evaluated. 



9. Onlap and Off lap: Onlap and off lap problems require the three- 

 dimensional approach. Subsurface studies permit determination 

 of rate, dimension, and trend of these depositional conditions. 



10. Miscellaneous Problems: Other subsurface problems confront- 

 ing the geologist and engineer in addition to those mentioned 

 above include: cementing, setting of casing, hole caving, fishing, 

 stabilization of drilling fluids, perforating, formational water 

 variations, porosity and permeability changes, coring, and test- 

 ing. 



Training of the Subsurface Geologist 



Courses in subsurface geology, as given in some universities in the 

 United States, vary according to geographic location, facilities, and instruc- 

 tional personnel. Prior to these courses the student should have a thor- 

 ough background in petrology, petrography, structural geology, field 

 geology, petroleum geology, mineralogy, stratigraphy, sedimentation, 

 paleontology, and geophysics. 



A sequence of subject material in a formal university course in sub- 

 surface geology is suggested. 



Lithologic Studies: Lithologic types including shales, limestones, 

 dolomites, sandstones, and other lithologic varieties should be examined 

 and studied under the binocular microscope. Each lithology should be 

 represented by chip fragments in a reservoir-type slide and viewed by 

 the student during instruction. This method is extremely applicable and 

 time-saving in preparing the student for subsequent well-logging assign- 

 ments. 



Well-Logging Methods: The various types of well-logging methods 

 should be briefly summarized. These methods should include lithologic, 

 electric, radioactive, drill-time, caliper, and thermal logging. The instru- 

 mentation, use, and limitations of these methods should be treated. 



Theoretical Electrical Profile Interpretation: After the student has 

 become familiar with lithologic and electrical relationships, he should be 

 required to plot from tabulated data (mimeographed) a percentage log 

 from which an interpretive log is prepared. On the basis of the latter 

 log theoretical electric profiles may be drawn. This problem demands 

 that the student think in terms of both lithology and its probable electrical 

 reflection. 



Preparation of Well Log: The student, once having become familiar 



