This question of the origin of petroleum has occupied the attention of many of the most noted scientific men of recent generations, but even down to the present day no one has devised an explana tion acceptable to all. Opinions remain still dia metrically opposed, with names carrying much weight arrayed on both sides. In general, the con troversy is between the chemists on one hand and the geologists on the other, although there are some few of each class who must be regarded as deser ters from the ranks.
The chemists believe that petroleum is of inor ganic origin, or, in brief, that it is the result of instantaneous chemical reaction between carbon and hydrogen coming in contact under special conditions underground. The famous chemist Ber thelot was the first to advance this idea. In his estimation an intensely heated metallic kernel in the center of the earth, carbonic acid and water coming in contact with these metals and with each other, were sufficient to produce the compounds of carbon and hydrogen found in petroleum. Most of the prominent chemists of Europe, among them the great Mendeleef, have accepted this theory with slight variations, and to justify their belief they point to very beautiful laboratory experiments in which artificial petroleum has been made.
On the other hand, a very serious objection to the chemical origin is found in the well-known fact that petroleum varies greatly both in physical and chemical character, whereas it is a fundamental principle of chemistry that a given reaction al ways produces the same compounds with the same character and the same composition. The adher ents of the chemical theory usually try to explain away this objection in an offhand manner by say ing that the variations are the results of differ ences in the strata through which the petroleum has passed since its formation. But from evidences found in the wells it is known that there has often been very little, if any, movement of oil through the rocks, while, even if there had been such movement, the rocks do not vary enough to account entirely for such wide differences as are observed within the limits of relatively small areas.
What may be called the geological theory is based on the facts observed in the study of the different formations where oil has been found. On the basis of these facts, geologists as a body regard petroleum as of organic origin, or, in other words, as the result of slow decomposition of organic re mains, animal or vegetable, which have been stored up in the rocks since the time of their formation. But, even among themselves, geologists can not come to any unanimous opinion, some holding to the idea that the organic remains were mainly de rived from plant life—partly water, partly marsh, and partly land species—while others maintain just as stoutly that it was essentially animal life, much the same as is found in the ocean to-day ; and many arguments are advanced to support the statements of both sides.
In the midst of all these conflicting views, it is hard to determine just which ideas are most clearly entitled to receive general acceptance. The pres ent tendency, however, is to accept without much question the geological theory of organic origin as being based on actual observation rather than on laboratory deductions. At the same time the num ber of advocates of the animal life theory is grow ing much more rapidly than is the number of those who hold to the plant life origin. It is not un likely that each is partly right: that while the larger proportion of petroleum deposits have been derived from animal remains, some particular de posits, as certain light-colored oils of Pennsylva nia, may be of vegetable origin. The variations in character and composition of crude petroleum are thereby readily accounted for as coming from differences in the animal or plant remains, while different degrees of heat, varying lengths of time in the process, and minor differences in rock mate rial may have aided in determining individual characters.
With very few exceptions, the accumulation of important petroleum deposits has been controlled by the character and attitude of the strata. In fact, the occurrence of commercially valuable de posits depends fully as much on the existence of suitable conditions in the rocks as it does on the presence of an adequate original source of supply. The first requisite for accumulation is the presence of a coarse-grained, porous rock to act as a reser voir in which the oil can be stored. The usual reservoir rocks are sandstone, whence the common term " oil sands," although certain limestones, and occasionally a pudding stone or conglomerate, serve the same purpose. This reservoir rock must be entirely covered by a fine-grained, nonporous layer, the impervious nature and absence of frac tures in this second layer controlling to a very large degree the ability of the reservoir rock to retain its store of oil. Fine-grained shales are the commonest and most perfect covers for these un derground reservoirs, and where such a combina tion of porous and nonporous strata exists in an oil-bearing region, the formation of a workable oil field usually results. In some cases, as in the limestone oils of Ohio, the reservoir rock is also the source of the supply, the oil being stored in the same strata where it was formed. In other cases, as in most of the Pennhylvania districts, where the oil is found in sandstone strata, the source of the supply is below the reservoir rock, and distinct from it.