ozokerite, etc. These substances are usually considered as being of organic origin and to have resulted from changes which take place in vegetable and animal tissues when buried and in most cases subjected to heavy pressure. A large part of the hydrocarbons referred to is thought to have been derived from animal organisms, an opinion which is sustained in an important manner by the fact that large stores of both petroleum and rock-gas have been discovered in rocks which were laid down before land vegetation is known to have existed. Marine algæ were present, however, so that it cannot be affirmed that the hydrocarbon of the earlier Paleozoic rocks came entirely from animal organisms. It is highly probable, however, that a large portion of the hydrocarbons stored in Paleozoic and later strata was derived from the animals whose hard parts occur so abundantly as fossils in the same or adjacent beds.
Fig. 35.—Ideal section showing favourable conditions for the storage of petroleum and gas.
Besides the concentration of carbon in plant and animal tissues and its change to hydrocarbons, there is a still further concentration necessary in order that stores of petroleum, gas, etc., shall be accumulated so as to be of economic value. This accumulation is dependent largely on physical conditions. The production of hydrocarbons from organic matter contained in sedimentary rocks, and particularly in shale, is going on in many regions, and probably nearly everywhere, especially when the soft parts of animals are buried in the rocks, but the petroleum, gas, etc., generated escape at the surface and pass into the air and are again widely disseminated, unless conditions are present which lead to their accumulation. The conditions favouring the natural storage of the substances referred to are cavities, or more usually porous beds, such
as sandstone, beneath impervious beds, such as clay or shale. The conditions are still more favourable when lateral as well as vertical escape is cut off, as, for example, when arches or domes occur. The most favourable conditions result when a bed of shale or other rock, as a, Fig. 35, from which hydrocarbons are being evolved occur beneath a sheet of porous sandstone or fissured rock of any kind, b, above which there is a close-textured, unfractured stratum, such as shale, c, and the series is bent along certain axes into upward folds or anticlinals. Under these conditions, as extended experience has shown, a well drilled at d should yield in succession gas, petroleum, and water.
The conditions for the production of petroleum, gas, etc., have been present on the earth since the first appearance of life, and reservoirs may have originated at any subsequent time. The oldest known reservoirs still charged with these substances that have been discovered occur in the earlier Paleozoic rocks, just above the formations containing the oldest known fauna. Important petroleum and gas fields in rocks of the Trenton period occur in New York, Ontario, Ohio, and Indiana. The Devonian rocks of Pennsylvania, New York, Ontario, etc., also yield large supplies of both oil and gas. Mesozoic rocks of Colorado, Wyoming, etc., are also rich in the concentrated hydrocarbon referred to, and on the Pacific coast, particularly in California, rocks of Cenozoic age are highly productive. Petroleum and gas may occur also in rocks more recent than the Cenozoic, but owing to the absence of reservoirs, and possibly the lack of sufficient time, no important accumulations are known in beds more recent than the Tertiary, unless they come from a deeper source in older rocks. The vast quantity of petroleum stored in the rocks of various ages in North America is indicated by the fact that in 1900 the yield from the wells of the United States was 63,362,704 barrels, and from Canadian wells about 280,000 barrels, making a total of nearly 64,000,000 barrels.
The stores of rock-gas are also enormous, as is indicated by the fact that a single well at Bairdstown, Ohio,
yielded over 17,000,000 cubic feet per day. In 1890 the average daily flow of the Indiana gas-wells was 779,525,000 cubic feet. The value of the natural gas consumed in the United States in 1900 was $23,606,463.
In the sedimentary rocks of North America there occur also extensive and valuable deposits of semifluid and solid hydrocarbons, such as maltha, asphaltum, albertite, grahamite, uintahite, etc., which have arisen, under the most plausible explanation thus far offered, from the concentration by evaporation of fluid hydrocarbons such as petroleum. The evaporation, particularly of heavy petroleum, leads to the formation of a solid residue, similar to asphaltum. In fact, there is no definite boundary between the lightest naphtha and the most coal-like asphaltum. They form a connected hydrocarbon series, analogous to the coal series.