28 Subsurface Geologic Methods 



basin of sedimentation, respectively. Tectonics is indeed the soul of the matter. 

 Therefore, the first breakdown of environments must be tectonics. 



The importance of facies development and interrelationships have too 

 often been neglected and minimized in favor of the structural impress in 

 selecting potential petroliferous areas. The definition and evaluation of 

 facies changes are as important in outlining favorable petroliferous areas 

 as are the structural anomalies developed within them. 



The question arises: What is the importance of evaluating facies 

 changes? There are several answers: 



1. Lateral changes commonly involve variations in porosity and 

 permeability, which control selective accumulation of liquid and gaseous 

 hydrocarbons. 



2. Selection of areas containing proper facies and facies relation- 

 ships may reduce exploratory costs for an oil company. The facies factor 

 has many times been more important in controlling oil and gas accumula- 

 tions than structural conditions. 



3. Paleogeologic data may be more accurately interpreted when 

 lateral variations and relationships of the sediments are properly inte- 

 grated. 



4. Before faunal and stratal sequences can be properly arranged 

 chronologically, a knowledge of facies relationships must be known. 



5. Variations in rock types may possibly control localization of 

 ore deposits. 



6. Migrating lithofacies boundaries may be incorrectly interpreted 

 as structural reflections. 



7. Changes in facies may complicate engineering problems, such 

 as tunneling, excavating, and estimating costs. 



Facies changes result from fluctuation of sea level, climatic varia- 

 tions, modification of topographies, diastrophic readjustments of the hin- 

 terland, changes in oceanic currents, and drainage patterns, erosional 

 cycles, readjustment in deposition basins, and migration of shore lines. 



When one is introduced into an unfamiliar province the approach to 

 the solution of stratigraphic problems is first to generalize the stratigraphy 

 and structure of the area and second to give consideration to the details of 

 the section. This stage is followed by repeated regeneralization and re- 

 detailing. Overemphasis of either generalizing or detailing may retard 

 progress or promote erroneous interpretations. 



Reliable stratigraphic information demands accurate field control. 

 Without adequate structural information, stratigraphic sequences and 

 facies variations of deposits cannot be properly allocated. An unrecognized 

 fault or nonrecognition of dissimilar facies equivalents may increase or 

 decrease the normal thickness of stratal sequence by thousands of feet. 

 The failure to recognize unconformities may introduce conflicting strati- 

 graphic interpretations. 



