GEOLOGICAL HORIZONS OF PETROLEUM.
It is generally supposed that rock-oil exists in fissures or cracks running across the strata and extending obliquely or perpendicularly towards the surface; but this theory is contrary to experience and the nature of petroleum. In the first place, oil is always found on certain horizons,—below a special sandrock in Venango, and at a uniform geo logical depth in other localities; and, in the second, if oil existed in fissures, it would long ago have ceased to be oil, and become solid bitumen from the escape of the volatile parts.
In all oil-producing regions the strata are nearly horizontal, but having a gentle incli nation and a basined shape; but all such basins are necessarily extensive, because the low angle; of dip increase the distance between the outcrops. The natural position of oil in the strata is similar to that of coal. It occupies certain horizons between the strata, perhaps irregularly, but always below heavy, close-grained sandrocks, which are impervious to the escape of oil, and almost impenetrable to the most volatile gas, even under the highest state of tension: were it not so, but little oil could ever accumulate beneath them.
Beneath each heavy sandrock of Venango, and Northwestern Pennsylimnia generally, is found a soft and yielding stratum of shale, slate, and mud. In these strata the gas and oil find a lodgement, as the first point of condensation. Arising through the rocks below in a gaseous condition, the nearer it approaches the surface the less volatile it becomes, and is arrested by the first impervious stratum.
The third sandrock of Venango, however, does not seal all the oil: the more volatile portions still find their way through the third and even the second sandstones, and form limited deposits of oil between them; but the lower or third rock seems to hold the great fountains of petroleum.
The stratum of oil thus formed may not occupy a perfect geological horizon, since the shale in which it is found varies in thickness, and the oil may find a cavity in any part of it,—at the bottom, in the middle, or at the top of the stratum. Though the drill must
invariably penetrate the covering sandrock, it may not tap the fountains of oil at that depth. It is possible that the depth of wells almost side by side may vary 20 or 30 feet, or more, to reach the same oil-formation. A stratum of oil cannot be of great thick ness. It may be only a few inches, or one and even two feet thick; but it may also be a mere streak, or the rocks may close or "pinch" it out entirely. Thus, the stratum may extend from ten to one hundred yards in width, and from a hundred yards to miles in length, or it may be even more limited or more extensive. It may change from a lower to a higher position in the shale, thus: and the auger may strike a flowing well at 4, only a limited pumping well at 3, though but a few yards distant, and nothing in the shape of oil at 5.
If oil existed in fissures running obliquely or perpendicularly to the strata, it might be found at almost any depth from the surface, and in the most irregular manner, with out regard to the sandrocks. But fissures in rocks existed naturally and originally as vents for the internal heat or gases of the earth, and generally extended to the surface; and were these the reservoirs of oil, we cannot see how the light naphtha could be re tained, since the volatile parts would escape through every crack or vent to the surface, and leave nothing but bitumen as a residue.
Figure 190 illustrates clearly the action of gas in producing the flow of petroleum from oil-wells. The stratum of oil, f, is always the lowest, while the gas, b, is always the highest, and occupies cavities in the shale invariably above those containing oil.
These cavities, as before stated, are irregular; they may exist one above the other, or they may be located at considerable distances apart, and yet communicate, since the oil is the result of the gas in a condensed state: therefore a communication must exist be tween the reservoirs of oil and the gas producing them.