364 The Philippine Journal of Science 1913 



has been removed by erosion so as to expose sandstone belonging 

 to the Vigo shale above the Bacau stage, and this sandstone 

 generally has not been found to be petroliferous. Thus, while 

 the overlap of the Canguinsa sandstone may conceal petroliferous 

 horizons, there is little direct evidence that these concealed hori- 

 zons supply the petroleum which appears in the Bacau stage 

 of the Vigo shale. 



Similarly, there is a chance that the unexposed basal portion 

 of the Vigo shale or a separate underlying formation is the 

 source of petroleum which has moved upward through cracks 

 and joints in the intervening beds to the Bacau stage. However, 

 a relatively great thickness of strata is exposed in the limbs of 

 the Central anticline between the Bacau stage and the lowest 

 beds of the Vigo which have been uncovered by erosion, and there 

 is but little evidence of oil in these intervening strata. Fresh 

 surfaces in the occasional fine-grained beds only show traces 

 of oil. Petroleum passing upward to the Bacau stage from a 

 reservoir in the hidden lower part of the Vigo might be expected 

 to leave traces along the outcrops of its passage through the 

 intermediate rocks. From the fact that the petroliferous shale 

 in the Bacau stage loses all evidence of oil after a short period 

 of exposure, it might be argued that the relatively coarse sandy 

 shales and sandstones between the Bacau stage and the lowest 

 exposed part of the Vigo shale would retain no oil at the surface. 

 In the data at hand, however, there is little evidence that the 

 petroleum -in the Bacau stage came there by diffusion from the 

 base of the Vigo shale or from an underlying formation. 



It appears, therefore, that while other accumulations of pe- 

 troleum may exist in the concealed members of the Vigo shale, 

 the petroleum in the Bacau stage probably originated somewhere 

 in that stage, although it may have migrated laterally through 

 the beds to the points at which it is now found. 



If exploration proves that no oil exists in this field, except 

 that which is evident in the Bacau stage of the Vigo shale, the 

 possible production of petroleum will be confined to the area 

 over which the strata are intact below the Canguinsa sandstone. 

 The oil content of the petroliferous shale in the Bacau stage 

 is probably low. Distillation of a sample, taken from a surface 

 outcrop and kept in a sealed package before testing, yielded less 

 than 1 per cent of oil. Beneath the surface where there has 

 been no chance for volatilization to take place the proportion 

 of oil is probably greater, but must still be relatively small 

 because of the close-grained nature of the shale. 



