586 p. G. CLAPP CLASSIFICATION OF PETROLEUM AND GAS FIELDS 



speculative, the general cross-section is presumed to be somewhat as in 

 figure 12. At the base of the upheavals and surrounding them in close 

 proximity the Tamasopo limestone and overlying formations form pockets 

 or places of catchment where large deposits of oil have accumulated. In 

 the Tamasopo limestone and the San Felipe beds these oil deposits were 

 presumably concentrated from surrounding portions of the same strata, 

 owing to the upheavals mentioned; possibly with the assistance of heat. 

 The presence of the oil accumulations surrounding the plugs is some- 

 times, although not always, evinced by large seepages of oil. Some cases 

 are known where the lower beds actually reach the surface and a true 



Figure 12. — Hypothetical Cross-section of a volcanic Plug in the Coastal 

 Plain of Mexico 



Showing occurrence of petroleum according to Subclasses IV {d) , V {d), and Class VII 



quaquaversal structure exists. AVhether this is common has been doubted,^'* 

 but it is certain that definite doming does exist surromiding some of the 

 plugs. At any rate it is a fact that where the plugs exist pockets of oil 

 have accumulated, and the conical plugs themselves may be considered as 

 quaquaversal structures. 



It would appear that large deposits of oil might be expected in the 

 vicinity of such intrusive masses in all cases where porous sands are over- 

 laid by a suitable cover to prevent the escape of oil. Where the imper- 

 vious covering or cap rock is unusually thick or without fractures, seep- 

 ages may be absent, although they exist in the vicinity of most of the 



^ E. De Golyer : The effect of igneous intrusions on the accumulation of oil in the 

 Tampico-Tuxpam region, Mexico. Econ. Geol., Nov.-Dec, 1915. 



