viii, a, 5 Pratt and Smith: Petroleum Resources 331 



cipal known oil horizon. It is not porous enough to afford a 

 reservoir in which oil might accumulate, and no oil has been 

 observed in it. Because of its compact nature on the other hand, 

 it would tend to confine any oil collecting below it. At several 

 promising drilling sites the Canguinsa sandstone must be drilled 

 through before the petroleum zone is encountered. 



Vigo shale. — The base of the Canguinsa sandstone is marked 

 by an unconformity, which is partly of a mechanical nature, but 

 may represent also a period during which the underlying forma- 

 tion, the Vigo shale, was subjected to erosion. The subject of 

 unconformities is discussed in connection with the geologic struc- 

 ture, page 337. 



The Vigo shale is the most extensive and the most uniform 

 series in the stratigraphic column of Bondoc Peninsula. The 

 beds belonging to this formation, although they are closely re- 

 lated in type to some of the overlying beds, constitute a separate 

 stratigraphic division which is readily distinguished. 



The type exposures in the valley of Vigo River consist of fine- 

 grained shale and sandy shale interstratified in thin regular beds 

 from 5 to 10 centimeters in thickness. Occasional beds of sand- 

 stone occur varying from 10 centimeters to 1 meter in thickness. 

 The fine-grained shale is gray, blue, or black, and is made up 

 almost entirely of clay. The sandstone is gray or brown, and 

 consists of unifornvmedium-sized, not completely rounded grains 

 of quartz, diorite, andesite, and metamorphic rocks. The sandy 

 shale is yellow or brown and of intermediate composition. 



There is an apparent transition from east to west in the 

 character of the Vigo shale. In the eastern limb of the Central 

 anticline, exposed in the valley of Vigo River, the formation is 

 predominately shale throughout, sandstone occurring only at in- 

 tervals. In the western limb shale predominates in the exposure 

 near the axis only, that is, the lower part of the series. Farther 

 to the west the sandstone beds increase in number, until in the 

 upper horizons they become more prominent than the shale. 

 The grain-size likewise increases in the upper beds, and small 

 pebbles occur, forming layers of sandy conglomerate. 



The blue or black, fine-grained shale in the Vigo formation 

 usually emits a slight odor of light oils upon fresh fracture, and 

 in some outcrops is highly petroliferous. The material loses this 

 odor and assumes a light gray color after it has been exposed to 

 the air and has become thoroughly dry. The petroliferous shale 

 forms a loosely defined stage in the upper part of the Vigo, which 

 will be referred to as the Bacau stage, although it cannot be 

 sharply delimited. 



