fairly vigorous well to give one pound open pressure in a pipe of this size. The largest well ever struck in Ohio, viz., the Van Buren well of Hancock county, gave six pounds open pressure. When the same well was brought down to a 4-inch pipe, the gauge showed an open pressure of 20

pounds.

Wells of less than one million feet per day will produce either no effect or too small an effect to be easily noted on a high-pressure gauge, and for such wells other means of measurement, as the water or mercury column or the anemometor, must be adopted.

In defining open pressure, a partial explanation of it has been at the same time given. It results from the volume of gas that escapes in agiven interval from the reservoir, and thus obviously depends on the general conditions and especially on the porosity of the reservoir. It is not connected with, or dependent on the closed pressure of the wells. A well may have a high closed pressure and a very low open pressure. Such a well will require a longer time to reach its maximum after being locked in. The gauge shows a very slow rate of advance. The gas, in other words, comes into the well slowly, which is the same as saying that the well is a small one.

It is the open pressure by which the flow of a well can be determined. Nothing whatever can be told of the production of a well by its closed pressure, taken by itself. Wells in any field are quite likely to hold the same closed pressure, no matter what their production may be. That is, when locked in, all such wells will reach the same figure on the gauge. One well reaches it instantaneously, or in a very short interval. In another, the pressure creeps slowly up for hours, but at last it gets to the same point that its great neighbor reached in a minute.

If the element of time be combined with the element of closed pressure, an approximation can be made to the production of a well. It is obvious that a well that gains 100, 200 or 300 pounds in a minute has a much larger production than a well that gains 5, 10 or 20 pounds in the same interval.

It is probable that an empirical relation can be established for facts of this character, but no tables are yet known that cover the ground. The only scientific system of measuring the flow of gas-wells, that is at the same time applicable to wells of all grades of production, is that which will be found in a subsequent chapter of this volume. It is based on the open pressure of the gas, and by means of it the strongest well that ever blew can be determined by a simple and single measurement that can be executed in a few seconds at most.

Both the closed and open pressure of wells have now been defined,

and the latter has been explained to a certain extent. Can an explanation be offered for the facts of closed pressure as well?

The subject is an interesting and important one, but it is confessedly obscure and difficult. There has been but little written upon it by those whose judgment would be worth most to us. The facts pertaining to the closed pressure of great gas-wells are among the most striking in the whole range of mining enterprise. To be appreciated, a high-pressure gas-well must be seen and heard. The gas issues from it with a velocity twice as great as that of a bullet when it leaves a rifle. Sets of drilling-tools, nearly 100 feet long, and weighing 2,000 pounds, are lifted out of a well 1,000 or 1,500 feet deep and thrown high into the air. The noise with which the gas escapes is literally deafening, exposure to it often resulting in partial loss of hearing on the part of those engaged about the well.

What is it that originates this indescribable force? Three answers have been offered, but no one of them, so far as is known, has been expanded into anything like an elaborate and balanced statement. They have been thrown out as suggestions rather than as full-fledged theories.

One answer is, that the rock-pressure is derived from the expansive nature of the gas. Solid or liquid materials in the reservoir are supposed to be converted into gas as water is converted into steam. The resulting gas occupies many times more space than the bodies from which it was derived, and in seeking to obtain this space it exerts the pressure which we note.

This view has, no doubt, elements of truth in it, even though it fails to furnish a full explanation. For the pressure of shale-gas, it may be that no other force is required. But the theory is incapable of verification, and we are not able to advance a great ways beyond the statement of it. Some objections to it will also appear in connection with facts that are presently to be stated.

The second explanation that is offered is, without doubt, more generally accepted than any other by those who have begun to think upon the question at all.

This theory is to the effect that the weight of the superincumbent rocks is the cause of the high pressure of gas in the reservoirs. In other words, the term rock-pressure is considered to be descriptive of a cause as well as of a fact. That a column of rock, 1,000 or 1,500 feet deep has great weight, is obvious. It is assumed that this weight, whatever it is, is available in driving accumulations of gas out of rocks that contain them, whenever communication is opened between the deeply-buried reservoir and the surface.

Is this assumption valid? Can the weight of the overlying rock work in this way?

Not unless there is freedom of motion on the part of the constituents of the rock, or, in other words, unless the rock has lost its cohesion and is in a crushed state. If the rock retains its solidity, it can exert no more pressure on the gas that is held in the spaces between its grains than the walls of a cavern would exert on a stream of water flowing through it. Professor Lesley has discussed this theory with more elaboration and detail than any other geologist, and has shown its entirely untenable character. (Annual Report Penna. Survey, 1885.)

The claim that the Berea grit or the Trenton limestone, where they are, respectively, oil or gas-rocks, exists in a crushed or comminuted state, is negatived by every fact that we can obtain that bears upon the subject. The claim is a preposterous one, but without this condition the theory fails.

The third theory advanced to account for the rock pressure of gas stands on a different basis from those already named. It appeals to water pressure in the oil and gas-rock, as the cause of the flow of both these substances, and in this reference, it directs us to principles and facts of familiar experience and every-day use. Every one is acquainted with the phenomena and explanation of artesian wells. By this theory gas and oil-wells are made artesian in their flow. In the porous rock that contains them there is always, outside of the productive fields, a body of water, and in almost every instance, salt-water. This water occupies the rock as it rises to day in its nearest outcrops. Communicating there with surface water or with rainfall, a head of pressure is given to the gas and oil that are held in the traps formed by the anticlinals or terraces into which the stratum has been thrown. The amount of pressure would thus depend on the height to which the water column is raised, in case continuous porosity of the stratum can be assumed. Defects in regard to porosity would abate from the total pressure on the oil or gas.

This, in short, is the third and last of the explanations offered of the rock-pressure of natural gas. There seems little reason to doubt that it is along this line that the true explanation is to be found, though it is too early to claim that a full account can now be given of all the facts involved.

One of the significant elements in the case is the salt-water that surrounds every oil and gas-field. When the drill descends into this outside territory, salt-water promptly rises in the well to the surface, or to a given depth below the surface. Sometimes, indeed, it overflows. Why does the salt-water rise ?

What other cause can be suggested than pressure from behind? The rise must be artesian. But just beyond the salt-water, on a slightly higher level of the rock, lies the oil pool. When that is reached by the drill, the oil flows out from the well. Will not the same cause that we found in active and unmistakable operation in the adjacent salt-water territory explain the flow of the oil from the second well? Is not this also artesian ?

In like manner, the pressure of the gas that is confined within the highest levels of the same porous rock can be explained, and thus one familiar cause that is demonstrably present in the field is made to account for the varied phenomena presented.

With the exhaustion of a gas-field or oil-field, these substances are followed up and replaced by salt-water. This is the common fate of gas and oil-wells, the death to which they all seem to be appointed.

tion:

Certain obvious inferences follow the acceptance of this explana

1. The supplies of gas and oil are seen to be definitely limited by this theory of rock pressure. If a salt-water column is the propelling force, it is idle to speculate on constantly renewed supplies. The water advances as the gas or oil is withdrawn, and the closing stage of the oil-rock is, as already pointed out, a salt-water rock.

2. Other things being equal, the rock pressure will be greatest in the deepest wells. The deeper the well, the longer the water column.

3. Other things being equal, the rock-pressure will be greatest in districts the gas or oil-rock of which rises highest above the sea in its outcrops. The 750 lbs. of rock-pressure in Pennsylvania gas-wells, as contrasted with the 400 lbs. pressure of Findlay wells, can be accounted for on this principle.

4. The rock-pressure of gas may be continued with unabated force until the end of production is at hand. Maintenance of pressure is no proof of renewal of supply. The last thousand feet will come out of a gasholder with as much force as the first thousand feet.

5. Where both oil and gas are found in a single field, the first sign of approaching failure will be the invasion of the gas-rock by oil, or of the oil-rock by salt-water.

CHAPTER III.

THE TRENTON LIMESTONE AS A SOURCE OF OIL AND GAS IN OHIO.

BY EDWARD ORTON.

The entire history of the discovery and exploitation of petroleum in this country has been full of surprises, both to the practical men engaged in the work and to the geologists who have studied the facts as they have been brought to light, but no previous chapter of the history has proved as strange and well-nigh incredible as the discovery and development which are now to be described.

No fact in this line could be more unexpected than that any notable supplies of petroleum or gas should be furnished by the Trenton limestone, which is widely known as a massive, compact and fossiliferous limstone of Lower Silurian age and of wide extent, constituting in fact one of the great foundations of the continent. But when required to believe that certain phases of this Trenton limestone make one of the great oil-rocks of our geological scale, one which produces from single wells 5,000 barrels of oil, or 15,000,000 cubic feet of inflammable gas in a day, it is hard to prevent our surprise from passing into incredulity. It is this remarkable history which is to occupy the present chapter.

A brief account has already been given of the Trenton limestone (page 4, etc.), and its character, extent and composition have been indicated. To these statements, a few others are to be added.

The formation ranges in thickness from 50 to 800 feet, and when the limestone is replaced in part by shales, the formation sometimes becomes 2,000 feet thick.

In New York it is divided into two divisions, viz., the Trenton

limestone proper, and the Black River limestone. The lower portion of

the latter is sometimes separated from the stratum under the name of the Birdseye limestone. This designation is derived from the occurrence of small crystalline points in the limestone. Both of these divisions belong to the true limestones as distinguished from magnesian limestone.

In Illinois and Wisconsin, there is, also, a two-fold division of the formation, but on a different basis from that adopted at the east. The