The theory of artesian and of spouting petroleum wells is entirely different. While the latter owe their operation to an internal pressure, due to gases accumulated within a confined space, the former are due to the pressure of a liquid which is flowing—a pressure caused by a sheet of water of unequal height; and they spout with so much the more force in proportion as the difference of level between the orifice and starting point of the sheet of water is greater.

Petroleum reservoirs, or pockets, contain, along with the petroleum, gases, salt water, sand, and foreign substances of varying nature. The liquids and gases in these pockets are often submitted to very great pressure. If we make an aperture in the pocket, there will occur, by reason of the tension, and according to the location of the aperture, a sudden exit of gas, petroleum, salt water, etc. Yet it may happen that as the sounding well has been bored through the upper part of the pocket, where the gases are accumulated, only the latter will make their exit without any trace of petroleum. Under such circumstances the appearance of inflammable gases at the surface indicates pretty certainly the presence of inflammable liquids in the region explored, and will justify further exploration or the fitting of suction pumps to the well holes.

It will be understood that a natural flow of petroleum will occur only so long as the pressure is sufficient, and that a pocket may cease to give mineral oil spontaneously, even though it may still contain large quantities of it. This is the reason why at present spouting wells are not abandoned when they cease to operate, but are worked by lift pumps. The three diagrams, 1, 2, and 3, will give an idea of the different configurations that petroleum pockets may present. In No. 1, as the well hole reaches the summit of the gas chamber, the gases alone will be forced to the surface by reason of the internal pressure, and not the slightest trace of petroleum will accompany them.

In No. 2, as the well ends at the side of the pocket, only a portion of the petroleum—that which is included between the dotted lines—will come to the surface.

In No. 3, as the well ends at the lowest extremity of the pocket, nearly the entire contents of the latter will be forced out naturally. It results from this that in petroleum exploitation the sudden appearance and disappearance of the spouting in no wise proves that the pocket is exhausted.—Science et Nature.