OIL-REGIONS OF NORTHWESTERN PENNSYLVANIA.
The oil wells of Venango and vicinity are more productive than those of any other region yet developed, and the geological formation of this portion of the Alleghany coal field would lead us to expect this result naturally.
First. The several oil-bearing strata are here brought into a comparatively small thickness by the thinning of the sandstones from the east to the west, and the absence of the heavy limestones which farther to the southwest overlie the Devonian oil-forma tion and greatly increase the depth at which they exist. As before stated, the upper oils are always the thickest, heaviest, and most valuable, because the more volatile parts escape when near the surface; the middle oils, or those which exist at a reasonable depth from the surface,—say from three to six hundred feet deep,—are the most abundant. because at this depth it exists as naphtha, and contains the greater portion of its hydrogen ; but at a greater depth—say from 1000 to 1500 feet—the hydro-carbons exist principally in a state of gas, which to the present has not been utilized. There may be exceptions to this depth in the West, since there we may expect heavy oils at a greater depth, on account of the lower temperature which always existed there.
Second. The oil-formations of Northwestern Pennsylvania lie along the northeastern outcrops of the Great Basin. Here the Devonian rocks approach the surface, bringing
their oils within a practical depth below the influence of the atmosphere which thickens, and above the chemical action which holds the hydro-carbons in a state of gas.
Third. The even, undisturbed, and horizontal position of the strata in this region is extremely favorable to the existence or preservation of the oil in its fountains, which are thus sealed for use. The fine-grained texture of the sandstones, and their solid, un broken spread, the close and tenacious strata of shales and slates, and the intercalating clays, prevent the escape of the gas or oil in exhausting quantities.
Fourth. The middle position of this region, between the extreme heat of the East and the low temperature of the West, was favorable to the original formation of oil ; and this we think one of the great secrets of the abundance of oils along the central portions of the Great Alleghany coal-field.
We might assign other reasons, but the foregoing are sufficient. It will be necessary, however, to explain more fully the last item, since this may account for the formation of oil, as well as its abundant existence in certain localities, and limited existence in others, within the Great Basin.