STRUCTURE AND CONDITIONS AFFECTING PRESENCE OF OIL. 73 
near the surface, or at least considerably above the oil-producing 
zones. 
It may be questioned whether the presence of water is essential for 
the accumulation of petroleum in the upward folds of the strata under 
the conditions presented by the Santa Maria and Lompoc fields. 
Here the oil tends to rise to the surface and form seepages wherever 
channels of escape are offered. This is probably not due to hydro¬ 
static pressure, as there is no evidence that the water tended to 
rise in the same way; and it is just the opposite of the tendency 
ascribed to oil by upholders of the anticlinal theory, which would 
result in the oil descending and gathering in the synclinal troughs on 
subsidence or removal of the water. In the fields under discussion 
the oil is always intimately associated with gas. There do not seem 
to be, as a rule, separated stores of gas and oil, but the two are inter¬ 
mingled, or at least closely brought together, so that one is not usu¬ 
ally found without the other, although gas is sometimes found alone. 
The oil exhibits a tendency to migrate, as shown by its original con¬ 
centration from widely separated points of origin, by its surface seep¬ 
age, and by the energetic way in which it rises in the drill holes when 
a source of it is tapped. This migratory faculty may be ascribed 
entirely to the presence of the associated gas, which would cause the 
oil to fill every crevice offering a point of escape or a point of lodgment. 
If this is granted, it is evident that the points of accumulation of oil 
will be determined chiefly by the presence of cavities, large or small, 
offering a place for it to gather. Anticlines, being points of fractur¬ 
ing and in some places opening out of the strata, would afford likely 
places for the oil to lodge in those beds subject to fracture and for it 
to be imprisoned by overarching impervious beds. 
Aside from ideas as to accumulation of oil after such a fashion, the 
writers have come to the conclusion that in this region many of the 
“oil sands,” so called, are not true sands, but zones of fractured shale 
or flint offering interspaces in which the oil can gather. Beds of sand 
in the Monterey are scarce and thin. Some of the oil-producing zones 
are very thick, amounting to tens or even hundreds of feet. The oil 
occurs chiefly in the lower portion of the formation, where brittle, flinty 
shale is abundant; and as it is a noticeable fact that wherever these 
hard, flinty layers appear at the surface they are usually much more 
contorted and fractured than the associated softer shales, which are, 
in general, only folded and not broken, it seems likely that the same 
fracturing and resultant formation of an ideal reservoir for the oil 
takes place in the depths as at the surface. Where it is so fractured, 
the shale occupies a greater volume than before, showing spaces some 
of which are open and others partially or wholly filled with chalcedonic 
or bituminous material. The unfractured beds are more or less imper¬ 
vious to the rapid migration of the petroleum, and so act as barriers 
to keep the oil in the porous zones. 
