THE STRUCTURAL CLASSIFICATION 559 



Popular Misconceptions of the structural Theory 



In presenting this classification it is necessary^ to repeat the note of 

 warning that favorable structures do not hold oil in every case. The 

 idea that every anticline or dome holds oil is as frequent and erroneous 

 as is the impression that all coal is of Carboniferous age, and as petro- 

 leum geologists we must do our best to correct it. 



We must also remember that an anticline is rarely symmetrical, and 

 that in all asymmetrical anticlines one flank is more favorable than is the 

 other or the exact crest. We must realize that unconformities exist, and 

 that even in the absence of unconformities strata are seldom parallel; 

 also that deformation in itself may suffice to prevent an axis from stand- 

 ing vertical. We must take full account of known and possible water 

 conditions, differences in porosity, etcetera. In short, we must acknowl- 

 edge that favorable structures are so numerous and complicated that pre- 

 dictions based on them must be made with the greatest care, taking into 

 account not only the structure itself, but all other geological and physical 

 phenomena involved. It is our duty, not only to ourselves but to our 

 clients, to make as fine distinction as possible and not to leave any person 

 with an idea that the solution in a new and unknown field is a simple one. 



The structural Classification 



formulation of the classification 



The object of the classification is to describe the various types of ac- 

 cumulations by grouping them into classes, each division of which follows 

 a special rule of structure and all of which have certain aspects in com- 

 mon. The classification, as elaborated to date, is as follows : 



Classiffcation of Oil and Gas Structures 



I. Aclinal or subaclinal structure. 

 II. Anticlinal and synclinal structures. 



(a) Strong anticlines standing alone. 



(h) Well defined alternating anticlines and synelines. 



(c) Broad geanticlinal folds. 



(d) Overturned folds. 



(e) Lenticular nature of the sands. 



III. Monoclinal structure. 



(a) Monoclinal noses. 

 (&) Monoclinal ravines. 



(c) Structural terraces or "arrested anticlines." 



(d) Lenticular nature of the sands. 



IV. Quaquaversal structures, or "domes." 



(a) Anticlinal bulges or "cross-anticlines." 



