Shales and Clays. Shales and clays indicate deep water deposi tion. They have a finer texture than sand, are more dense and compact, and are so nearly impervious to the passage of oil that only rarely are they a source of it. However, as will be shown later, they do play an important role in the accumulation of bodies of oil and it is seldom that wells are drilled without penetrating wide bodies of these materials. When subjected to the influence of heat and pressure they may be altered to the form of slate, which is also frequently ,met in drilling. Soft shale and clay are often designated as 'gumbo' by drillers while slate, or any other hard substance that impedes the progress of the drill is known by the broad term of `shell.' In the various oil fields, different clays and shales become known to have certain features by which they may be distinguished, and the knowledge of these beds and their relation to each other and to the productive measures is often of value as a guide in drilling a new well.
Limestone. Beds of limestone consist of calcium carbonate par ticles with usually a cement of the same material, although the term limestone is generally applied as well to dolomite, a form in which part of the calcium carbonate is replaced with magnesium carbonate. It occurs often in exceedingly wide bodies, and is the 'source of petroleum in the Canadian fields of Ontario, in Ohio and Indiana, and is the main: productive body at the Spindle Top fields in Texas. Wells drilled in these fields are frequently 'dynamited with nitro glycerine in order to loosen the formation and extend the zone from which the oil is drawn.
Gravels and Conglomerates. These are composed of rounded pebbles of all sizes with collections of finer material occupying the voids between. Like the sands, they have a shallow water origin and their properties of texture and porosity bear similar relations to the collection and retention of bodies of oil.
Origin of Oil.
The invariable association of gas with oil, although the lat ter may sometimes form alone, seems to establish the fact that they have the same or a similar origin. Two general classes of theories as to the origin of petroleum have been developed, known as the inorganic theory and the organic theory, and while these have in turn been subjected to many interpretations, by as many theorists, the fundamentals only of each will be given below. The inorganic theory has been put forward by chemists and is, in a general sense, that surface waters pass to the heated interior portions of the earth, where they are converted into steam and combine with carbide of iron to form the hydro-carbon products ; these are then forced back to or near the surface by the force of the steam generated. Geological
developments, however, fail to substantiate this theory.
The organic theory ascribes animal and vegetable matter as the source of petroleum, and holds that this matter has been subjected to a slow distillation while covered so that no air was present. It accords more nearly with the facts of the occurrence of crude oil and is the generally accepted theory. The scattered distribution of oil, its almost invariable association with sedimentary rocks either containing or, closely situated to, fossils, and the fact that ordinary fish oil may be distilled so as to yield a number of the petroleum products, all seem to point towards petroleum having originated from some form of life the remains of which have been subsequently heated without access to air' and thereby distilled.
The trend in the more recent discussion of this subject has been in the direction of placing vegetable rather than animal remains as the principal source of the oil.* The immense amount of animal matter that would be required to supply the material and the present day conditions that may be noted in many parts of the world where vegetation accumulates in huge quantities in marshes, lagoons, and swamps are cited as evidence pointing in this direction. This accords also with the fact that oil is usually found in sands and that these are shallow water deposits.
Relation of Rock Structure to the Occurrence of Petroleum: It is evident that when material has been eroded and transported to where it is to be deposited, the deposition will not be uniform but that the coarser and heavier bodies will sink first, leaving the finer particles in a longer period of suspension. For this reason sands and gravels imply shallow water deposition while the more comminuted materials that form the shales and clays remain in suspension and are transported farther from shore so that they are deposited at greater depths and in more quiet waters. In the course of time these become covered with further depositions, the weight of the overlying strata causes the lower measures to become more compact and rock-like, and there are built up wide bodies of strata horizontally placed, or with only a slight inclination. During this period the shore line may advance and retreat many times, so that what was deep water becomes shallow, resulting in a bed of sand being deposited on top of a layer of clay, and vice versa (Figure 14). Eventually the constant effort of the internal forces at work in the earth's interior may alter the position of the entire mass, or portions of it, and tangential stresses may distort it by causing it to crinkle and bend into arch-like folds.