QUANTITIES.
There are certain conditions, as before stated, necessary to the existence of oil in available quantities and position. These are, briefly—simply considering the lithological and topographical features : first, uniformity of stratification ; second, horizontal posi tion; third, the absence of fissures, dikes, and crevices for the abundant escape of gas ; fourth, closeness of texture and stratification in rocks, slates, shales, and clays ; fifth, a medium depth : if too high, all or most of the petroleum will have evaporated ; if too low, it will be difficult of access and only exist in a state of gas.
It has been found, and will always be found, that the most available petroleum exists at a depth of from 300 to 700 feet. If found higher, it is always in limited quantities and heavy; but if lower, it will be very light and gaseous.
It has been proven by a thousand oil-wells in Northwestern Pennsylvania that the distance from the millstone grit of the coal measures to the most abundant reservoirs of petroleum is about one thousand feet. But there most of the productive wells have been started from four to six hundred feet below the coal measures. The same oil formation exists beneath the coal-field to the south, but the coal comes down gradually to the level of the rivers and streams, and eventually passes under them in that direc tion. In such localities the depth of the oil will be from 1000 to 1500 feet ; and, as before observed, at such depth the hydro-carbons exist in a state of gas. Continuing still south and west, the Carboniferous limestone increases rapidly in thickness, and divides the Devonian oil-formation from the coal measures. It is only three feet thick on the Alleghany escarpment, two hundred feet beneath the Ohio at Wheeling, and one thousand feet thick under the Great Kanawha. This increase of the mountain limestone places the region of the second oils in a southwestern direction beyond available depth, under present developments ; but the time will come when both the means to reach those deep hydro-carbons and the means of utilizing their gases will be obtained.
The most available regions of petroleum in the great Alleghany coal-field will, there fore, be found, where the Devonian rocks are most accessible, within the wide and un dulating plateau before mentioned; and perhaps it will be found in greater quantities along their eastern margins than on their western outcrops.
Where the anticlinals sink beneath the coal measures, and yet preserve to some extent their shape, it is manifest the oil-rocks will be nearer to the surface than in the basins, and, consequently, bring the oil within available distance. This feature is repre sented at g, in figure 188, and may be studied at what is erroneously called the "Great Upheaval" on the Little Kanawha.
But within certain portions of the coal-field, where the base of the barren measures forms the bed-rocks of the streams, the upper petroleum may be found in available quantities in wells of 600 or 700 feet deep, or just beneath the millstone grit. At lower geological levels—say from the lower Freeport seam E, or even from the great seam of Karthaus, B—the upper oils may be, and, in fact, are, found productive at less depth. The upper oil-rocks produce most of the petroleum on the Little Kanawha and in all the region between that point and the second oil-rocks of Northwestern Pennsylvania, all the oil of Ohio, the Great Kanawha, and Western Kentucky, simply because the second oil-formation exists at a great depth, and the auger has not yet penetrated it, except on the Great Kanawha, where the salt-makers have reached its gaseous fountains, and there its flow has been terrific, as gas.