268 The Philippine Journal of Science 1915 



sibly also Canguinsa clay-tuff — into the fractured and brecclated 

 zones resulting from the intrusions. 



There is no way of ascertaining what quantity of petroleum 

 may be obtained from a petroleum field except by the actual 

 drilling of wells, and speculation in advance of all exploration 

 can have little value as an estimate. In the study of the Tayabas 

 field it was observed that petroleum reached the surface near 

 the top of the Vigo shale only, in a series of close-grained beds 

 from 50 to 100 meters in thickness, and the inference was clear 

 that if the petroleum were confined to these beds in which it 

 appeared at the surface there could be little hope of a quantity 

 of petroleum comparable with the yields of the larger fields in 

 America. In other words, the porous beds necessary to serve 

 as reservoirs for large petroleum accumulations are not extensive 

 or, at any rate, not prominent, in Tayabas. 



In Leyte the situation is similar, although petroleum appears 

 to be present at horizons lower in the Vigo shale than are the 

 seepages in Tayabas, and the quantity of petroleum represented 

 by the various deposits of residual bitumens is much greater 

 than there is direct evidence of in the surface indications in 

 Tayabas. But the petroleum from which the bitumen deposits 

 in Leyte are derived may have been driven from the petroleum- 

 bearing rocks by the action of heat from intrusions, a process 

 which would have removed petroleum from the Vigo shale or 

 the Canguinsa clay-tuff very efficiently. Similar intrusions in 

 the Vigo shale in Tayabas might have caused equally extensive 

 surface showings. If the possible agency of intrusions in the 

 concentration in Leyte of quantities of residual bitumens great 

 enough to demonstrate the former presence of large volumes of 

 petroleum be admitted, the presence of these bitumens does not 

 prove that drilling would result in the recovery of equally large 

 volumes of petroleum, even though the major part of the original 

 supply is still held in the rocks. Drill holes simply permit the 

 flow of petroleum and are most effective when they penetrate 

 saturated, porous beds, while intrusions would have forced the 

 petroleum out of the adjacent beds — porous or close-grained, 

 saturated or only partly saturated. 



It is improbable that wells drilled into the close-grained Vigo 

 shale either in Leyte or in Tayabas will yield the large flows 

 obtained in other fields from extensive lenses of porous sandstone. 

 There is in Leyte, however, the chance that pockets of petroleum 

 in the broken zone surrounding an intrusion (assuming that the 

 intrusions have in reality been effective in concentrating the pe- 

 troleum) may have been preserved by the sealing up of passages 



