5&} F. G. CLAPP CLASSIFICATION OF PETROLEUM AND GAS FIELDS 



{!)) Monoclinal bulges. 



(c) Closed saline domes. 



(d) Quaquaversal structure caused by volcanic plugs. 



(e) Perforated saline domes. 



V. Contact of sedimentary and igneous rocks. 



(a) Contact of sedimentaries with volcanic plugs. 

 (&) Contact of sedimentaries vp^ith dikes. 



(c) Contact of sedimentaries w^itli intrusive beds. i 



' (d) Contact of sedimentaries with other igneous rocks. 

 VI. Strata dipping unconformably away from an old shoreline, 

 VII. Crevices of igneous rocks. 

 VIII. Crevices of sedimentary rocks. 

 IX. Faults. 



(a) Upthrow side. 

 (&) Downthrow side, 

 (c) Overthrusts. 

 X. Sealed in by bituminous deposits. 



CLASS I— FIELDS IN ACLINAL OR SUBACLINAL STRUCTURE 



The term acline is defined by Webster's Dictionary as "Without in- 

 clination; horizontal." It differs from "acclinal," which means leaning 

 on another stratum. True aclinal formations are rare in geology. There- 

 fore, though proposed by Johnson and Huntley as a part of a summarized 

 classification, it does not need consideration here, except to explain that 

 the main cause of oil and gas accumulations is some sort of inclination 

 and folding. Where these do not prevail the oil, gas, and water remain 

 in their original unassorted state, and we have no pooh of commercial 

 value. ' A corollary to this principal has been observed in many localities 

 where the sands are nearly, though not absolutelyj flat, and a large num-'; 

 ber of wells get traces of oil and gas, while little in quantity exists at an;^' 

 particular point. , . , . 



Properly speaking, we may define subaelinal beds as those approxi- 

 mately flat, sometimes not dipping over 10 to 30 feet per mile, too slight 

 a dip to fully separate the oil and gas from the accompanying water. 

 Occurrences of gas or oil in such regions are generally mere showings, 

 encouraging to a prospector, but seldom resulting in real production. The 

 Electra pool in Texas is, however, mentioned by Johnson and Huntley as 

 an example of aclinal structure, the maximum dip being only 15 feet per 

 mile. 



GLASS II—FIELDS ASSOCIATED WITH ANTICLINAL AND SYNCLINAL 



STRUCTURES 



General discussion. — This is the class of oil accumulation with which 

 we are most familiar. It is generally supposed to predominate in a 



