X, A, 4 Pratt: Petroleum and Residual Bitumens 275 



heavier, associated water. Differences in the elevation of a 

 given bed are usually due to folding, and thus it is that petroleum 

 has so often been encountered in the arches or anticlines of folds 

 as to give rise to the term "anticlinal theory" to designate this 

 interpretation of the principles of petroleum accumulation. 

 Many other causes, of course, may result in the entrapping of 

 petroleum in the higher portions of porous beds: for example, 

 a dike of impervious rock may cut through beds tilted in one 

 direction and confine petroleum migrating through the porous 

 bed from below; or the same thing may be accomplished by the 

 sealing of the pores at the surface of tilted beds by the residuum 

 left from escaping petroleum so that the rest of the petroleum 

 is retained. In its widest sense the anticlinal theory accounts 

 for the petroleum accumulations in a majority of the productive 

 petroleum fields. 



Petroleum seepages occur along the anticlines both in Taya- 

 bas and in Leyte, indicating that the anticlines are points of 

 accumulation in these fields as they are in Sumatra and Japan, 

 in which countries large commercial productions are obtained 

 from formations of the same age and general character as the 

 Philippine petroleum-bearing rocks. 



Intrusive rocks are generally considered as an unfavorable 

 indication in connection with prospective petroleum fields, for 

 the reason that by their intrusion these rocks break up and 

 shatter the strata which they penetrate, destroying the original 

 structure and permitting the escape of any accumulated petro- 

 leum. However, the history of the development of parts of the 

 Mexican petroleum field, which has recently attained an enor- 

 mous production, shows that in this one instance intrusive rocks 

 have not been injurious, but have actually accomplished the 

 concentration of the petroleum, displacing it from buried strata 

 and providing a reservoir for its accumulation in the openings 

 of the shattered zone surrounding the intrusion. Intrusions 

 have become centers of accumulations, seepages occur near in- 

 trusions, and wells drilled in the vicinity of an intrusion have 

 repeatedly been successful. 



In determining whether drilling in Leyte is warranted and, 

 if so, where test wells should be located, data should be sought 

 which will tend to show whether the igneous intrusions have 

 merely dissipated the petroleum or have also promoted the ac- 

 cumulation of petroleum under favorable circumstances ; whether 

 there are accumulations of petroleum in structurally favorable 

 areas independent of the intrusions ; or whether both these f ac- 



