A third factor, which has aided in most large accumulations of petroleum, is the existence of folds in the strata, arches and troughs which have resulted from the shrinking and wrinkling of the earth's crust. Practically all the great oil fields of the world show that the important deposits are found under the roofs of the arches, or anticlines as they are called, because the oil is much lighter than the salt water occurring in the same strata with it, and hence rises to the highest points pos sible under the arching, impervious shale cover. This anticlinal theory of occurrence has often aided materially in determining the probable limits of a known field, and in the discovery of new fields and pools.
It may seem incredible that such enormous quan tities of petroleum as have been yielded even by single wells can be secured from a rock which ap parently is perfectly solid throughout. This fact more than anything else probably has fostered and kept alive the popular idea that the oil must come from fissures and cavities; for, it is argued, how else would there be room for so much oil? If a piece of the reservoir rock is examined under a strong magnifying glass, however, millions of tiny spaces appear between the different sand grains, and water will find its way through thick pieces in a com paratively short time. Such a porous, oil-bearing rock may very easily contain one tenth or one eighth of its bulk of petroleum, while some " oil sands " of the Russian fields have been calcu lated to contain not less than one fifth their bulk of petroleum. So innumerable are these pores in the rock that removal of the oil goes on readily enough, occasionally with enormous discharge for a time, without the existence of any definite chan nels to facilitate the process.
Both the storage of the oil and its discharge from the well are aided by the fact that the oil usually exists under pressure. The outward mani festation of this pressure appears most strikingly in the great flowing wells, sometimes yielding thou sands of barrels of oil daily. It was originally supposed that the pressure was due to the weight of the strata lying above the oil-bearing layers, but such an explanation is impossible. Even a com paratively weak rock will not crush under a weight of several hundred tons per square foot, or six to eight times the weight of the overlying rocks, at a depth of 1,000 feet. It has also been sug gested that the pressure is due to the " head " of water in the porous strata, as in artesian wells. But the facts observed in a multitude of instances do not show any parallel to the action of artesian wells, since the pressure steadily decreases almost from the moment the first discharge begins.
The most plausible explanation is that the pres sure is due to imprisoned natural gas which is almost universally associated with petroleum. This theory is supported by the fact that, as the gas pressure is relieved by escape to the surface, the flowing gradually ceases, and can rarely be induced again. The greater the pressure, however, the larger the amount of oil which can be stored in a given space in the rock and the larger the yield when the reservoir is tapped by a well. This ex planation readily accounts for the most phenom enal yields ever secured, without presupposing the existence of cavities of any sort.