572 r. G. CLAPP CLASSIFICATION OF PETROLEUM AND GAS FIELDS 



Actual determinations of the structure of the Clinton sand under exten- 

 sive areas have shown that where types (1) and (2) intersect, as in many 

 localities in the Bremen pools of Ohio, the largest accumulations of oil 

 are found. 



It is evident that fields of commercial importance will not commonly 

 occur in regions of 'plane monoclines any more than they will in absolute 

 aclines, since no factors of separation exist, with the one reservation, 

 that if the sands contain water, as most sands do at some locality or 

 other, an oil pool is likely to rest on it. However, some degree of in- 

 clination is necessary to cause the separation, and it is found in practice 

 tliat where this dip is less than half a degree the separation is so incom- 

 plete as to cause few, if any, commercial pools. Manifestly, the only way 

 to locate an oil pool on a plane monocline is to drill for it, since the sur- 

 face structure will afford us no aid. 



In the great monocline of central Ohio the water level has never been 

 found in the Clinton sand, although wells 4,000 feet deep have been 

 drilled. It is believed by geologists and the scientific oil men that when 

 this water is ultimately found, somewhere beneath eastern Ohio, a large 

 pool of oil will be found resting on it, similar in its trend to the great 

 central Ohio gas field. 



Suh class III (a) — Monodinal noses. — Attention was first called to the 

 monoclinal nose type, but without any particular name, by the present 

 writer in 1910.^^ This type of structure is very common in the gas fields 

 of central Ohio, and figure 8, from a bulletin on the Wooster field,^^ will 

 illustrate it. Examples are also frequent in the N'orth Texas fields, and 

 unfortunately have been confused with anticlines by some undiscriminat- 

 ing persons. 



This type may be considered as a less prominent form of Subclass 

 III(c), in that the terrace is not a well defined one. While it has been 

 noticed by the writer mostly in Ohio and Oklahoma, a number of ex- 

 amples have been reported by Gardner and others in Kentucky. 



Subclass ITI(b) — Monoclinal ravines. — The term "structural ravine" 

 was perhaps first used by the writer in 1911,^^ having exactly the same 

 relation to an inclined sand as a topographic ravine would have in a 

 sloping hillside. In the revised classification the term is changed to 

 "monoclinal ravine," as being somewhat more specific. 



Subclass III(c) — Structural terraces or ''arrested anticlines." — Ter- 

 race structure was first described by Orton in 1866.^^ The terraces de- 



^ Economic Geology, vol. v, no. 6, 1910, p. 508, fig. 53. 

 38 C. A. Bonine : Bull. 621-H, U. S. Geol. Survey, pi. xiii. 

 ^ Econ. Geol., vol. vi, no. 1, p. 10. 

 38 Edward Orton : Science, vol. 7, p. 563, 



