98 ALASKAN MINERAL RESOURCES IN 1906. 
The origin of petroleum may be explained according to one of two 
theories. The oil may be of organic origin, having been derived from 
animal or vegetable matter which was associated with the mineral 
constituents of the rocks at the time they were deposited, or it may^ 
be of inorganic origin, having been formed by the chemical action of 
water on the formerly unoxidized mineral constituents of the rocks. 
The prevalent scientific opinion is in favor of the organic theory 
for the origin of the larger and more widespread accumulations of 
petroleum. 
The movement of petroleum in the rocks is controlled by four fac- 
tors — the direct action of gravity, capillary attraction, the presence 
of water, and gas pressure. 
The effect of the direct action of gravity is to cause oil to go down 
as far as the rocks are porous, dry, and not too warm for the oil to 
exist as such. It will sooner or later be stopped in tins downward 
movement by an impervious stratum (either a bed of close-textured 
rock or a bed filled with water), and will then move laterally along 
the upper surface of thai stratum to its lowest point, where it will 
accumulate. 
The effect of capillary attraction is to cause the oil to be diffused 
throughout the rocks in all directions, provided the rock is dry anch 
of the right texture to permit capillary movement. The directions in 
which it will move will be controlled by the distribution of porous 
rock and will be modified by the other factors here discussed. 
The presence of water causes an upward movement of the oil. Th 
essential conditions for such movement are a porous rock containin 
both water and oil and a lower limit beyond which the water can no 
go. The water, because of its greater density, seeks a lower level tha 
the oil and forces it upward until either the demand of all the wate: 
for space is satisfied or the oil is checked in its upward movement by 
an impervious stratum. In the former case the oil rests on the sur- 
face of the water in a state of equilibrium; in the latter case ii is 
confined under pressure with a potential upward force. 
Gas pressure tends to drive the oil in any unblocked direction 
The requisites for oil movement caused by gas are the presence of gas, 
either in a contiguous body to the oil or being given off from or 
within the oil, and an impervious bed above the gas through which it 
can not pass. The gas then tends to accumulate on the upper surface 
of the oil and to force the oil downward in the direction of least 
resistance, which may either be vertical or have a lateral component. 
The oil would already have been in the lowest available space, and so 
further downward motion implies the displacement of water. The 
motion continues until there is equilibrium between the expansive 
pressure of the gas and the hydrostatic pressure of the water. Th€ 
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