644 



M. L. FULLl^R APPALACHIAN OIL FIELD 



principal retaining agents are listed in tlie following table, in wliich are 

 given confining agencies in oil pools other than structural limitations : 



Table Y. — Confining Agents in Oil Pools {other than structural Limitations) 



Confining _^ 

 agents. 



Covers (vertical f" Clays and shales, f Regular contacts. 



factors of lim- J Marls and lime- J Solntional irregularities of 

 1 stones. I contacts. 



L Igneous sills. L Unconformable contacts. 



itation — exter- 

 nal). 



Lateral factors of 

 limitation. 



f Thinning out of beds. 

 Lateral change in grain. 

 Lateral change in composi- 



^Internal factors. <^^ LateTal change in cementa- 

 tion. 

 Limits of action of solution. 

 Asphaltic or paraffin closure. 



r Erosion- 

 Unconformities. \ al. 



(, Overlap, 

 r Opposition of im- 

 FjmUs J pervious beds, 

 iauits.^ Crushed or shear 

 [_ zones. 



{Depositional mat- 

 ter. 

 Selvage. 



Igneous dikes, plugs, irreg- 

 iilar sills, etc. 



^ External factors. 



Of the covers, shales are the most common and important in the Appa- 

 lachian field. Igneous sills have never been encountered in this province, 

 but dikes are known in the midst of the oil field and sills may occur. Of 

 the internal confining agencies, lateral changes of grain are the most im- 

 portant. Almost nowhere is a single bed productive, without at least 

 local interruptions, for more than a few miles. Usually it is a change 

 toward tightness or fineness of grain that marks the limits of produc- 

 tivity. 



Cementation is a close second to grain in determining the extent of 

 productivity at a given point. Variations in cementation, either original 

 or due to solution, undoubtedly affect the location and extent of the oil 

 accumulations in many of the Appalachian sands. 



The pinching out of lenses is another limiting factor of great impor- 

 tance. Unconformities at the tops of producing sands undoubtedly exert 

 a considerable influence on the distribution of oil, at least locally. Closure 

 of sands by asphaltic or paraffin residues are not common in the Appa- 

 lachian field, bnt may occur in some instances. 



