598 F. G. CLAPP CLASSIFICATION OF PETROLEUM AND GAS FIELDS 



composition that the locality has suffered too great metamorphism, but 

 some of the deposits constitute indications of oil. 



To illustrate the importance of bitumen dikes as indications of the 

 former presence of petroleum and natural gas, it may be said that the 

 grahamite dike of West Virginia is in the center of one of the greatest 

 oil and gas regions in the world; that the albertite of New BrunswickWs 

 only a few miles from the Stony Creek gas field; that the grahamite 

 dikes of Oklahoma are within a few miles of known oil fields, and that 

 the uintaite dikes of Utah lead in a general direction toward oil, which is 

 found across the boundary in Colorado. 



Other Conditions than Structure 



The mistake must not be made of supposing that any one of the types 

 of structure is a positive indication of an oil field. Many other condi- 

 tions hold true, as we all know. After the structure has been determined, 

 it is necessary to learn (1) whether suitable sands with impervious covers 

 exist, (2) whether they are dry or wet, (3) whether there is any probable 

 source of oil or gas, and (4) whether the region shows evidence of too 

 great metamorphism, etcetera. 



Some persons have intimated that the structural classification is inade- 

 quate and hence of no value. To this criticism its propounder will merely 

 reply that it has proved of inestimable value to him and to many other 

 persons inside and outside of the profession. It is, of course, far from 

 perfect or complete ; yet some sort of a classification is needed, and noth- 

 ing has yet been suggested which comes near being a substitute. That 

 proposed by Johnson and Huntley,^^ in which the structures are arranged 

 in only four main classes, may offer some advantage by its apparent sim- 

 plicity, but no other formal classification is known. 



There are, however, several particulars in which fields appear to depart 

 from the ordinary structural principles, namely : 



1. As pointed out many times in structural treatises, the determining 

 factor in the distribution of gas, oil, and water in any pool is not the 

 general plane of the bed, but the undulation of the surfaces constituting 

 the roof and floor of the reservoirs. 



2. Lenticularity of the sands. In a strict sense of the word, of course 

 all oil sands are lenticular, though some extend continuously hundreds 

 of miles with little change in character. An example of this persistency 

 is the Clinton sand of Ohio. 



88 Principles of oil and gas production, 1916, p. 63. 



