OIL GROUPS.

As the experience of the last thirty years has abundantly shown, an oil or gas series always consists of two elements, viz., a porous rock, or reservoir, overlain by a close and finegrained impervious rock or cover. A third element must always be added to make out the logical series, viz., an underlying or associated source of oil and gas. It is obvious that the last-named element is first in order and in importance, but for reasons already given in part, and for others that are not hard to find, practically we have less to do with it than with the two former elements. It will be borne in mind that the sources of petroleum are well-nigh universal, and also that they have no economic value, and are therefore seldom penetrated by the drill. The search generally terminates in the reservoir. The great sources of the Ohio scale are, as already implied, shales and limestones, both more or less bituminous. These sources have done their work wherever large accumulation is found, and where no accumulations exist the petroleum occurs, as already shown, in large but valueless stocks distributed through the body of the strata.

THE RESERVOIR.

The reservoirs must be porous rocks. In all of the experience in the great fields of Pennsylvania and New York, the rocks in which the large stocks of oil and gas were found were, without exception, sandstones or conglomerates. To them the driller early gave the name of "oil-sands," and this name is in universal use. The grain and thickness of these sandstones are found to be important factors in their production. Other things being equal, the coarser the grain and the thicker the stratum, the greater is its production. found to be. Mr. J. F. Carll, of the Pennsylvania Geological Survey, our highest authority in regard to petroleum production, has shown that an oil-sand can hold one-tenth of its bulk of oil, and he believes that it may contain under pressure as much as one-eighth of its bulk. This would give 11⁄2 inches of oil to every foot of the oil-sand.

Taking the most productive portions of the latter in the Venango field to be fifteen feet, we find in that district a possible capacity of 9,600,000 barrels per square mile, an amount, it is needless to say, vastly in excess of any production ever known.- Second Pennsylvania Survey, Oil Regions," III., pp. 252

53.

The driller places great reliance on the oilsand, and learns to draw conclusions and make forecasts from its character more than from any other single element that he en

as porous, apparently, as the sandstones and conglomerates of the Pennsylvania series, this character being due in the limestone to the imperfect interlocking of the dolomite crystals. The dolomite constitutes but a small portion of the Trenton limestone in which it is found. The normal character of this great sheet is that of a true carbonate of lime, but it appears that, in a limited territory, the upper portions of the stratum have been transformed into dolomite. The transformation seldom extends more than a score or two of feet below the surface, and is often confined to five or ten feet. Sometimes a cap of true limestone, five or ten feet in thickness, overlies the dolomite, and sometimes the latter occurs in two or more sheets, separated from each other by the normal rock. The Trenton limestone is not itself a porous or reservoir rock in any sense of the word. It is only these replaced beds that have this character.

Besides sandstones and limestones, shales also serve to a small extent as receptacles of accumulated oil and gas in Ohio. The character of the containing rock in these cases is not well known. Generally, the gas is of light pressure, but it is a fairly persistent supply that is found in these rocks. The belt of shales along the shore of Lake Erie gives the examples of this sort of accumulation and supply. These shales, where productive of gas, are found to consist of hard and light-colored bands, interstratified with dark bands, the gas appearing to be found when the harder bands are penetrated. The production of oil from these sources is always small, but, as already stated, fair amounts of gas are sometimes derived from them.

Petroleum and gas are not the only substances that are found in these reservoirs. Salt-water is almost an invariable accompaniment of both. The oil-rocks are salt-rocks as well, in some parts of their extent. The distribution of these three substances in the same stratum is connected with facts of structure, as will presently be shown. These reservoirs have been described as porous of necessity. The porosity insures a large amount of lateral permeability, a fact of great importance in the distribution of these substances. The reservoir is often common for large areas. All the wells in a field may find the same pressure of gas or oil, even though their production may be very unequal.

THE COVER.

Inasmuch as the three elements-source, reservoir, and cover-are all indispensable, it is not necessary to compare their relative inportance. It is, however, true that the first and second conditions of accumulation are met more frequently than the third. The cover of every productive oil-rock is a large body of fine-grained, impervious clay shale-the finer and more nearly impervious the better. Whenever such a body of shale is found in the Ohio scale, the rock directly underlying, if a sandstone or limestone, is

found to contain, in some portions, accumulations of gas and oil. The stocks may be too small to be valuable, but the presence of the shale cover seems to insure some concentration in these situations. There are three points in the Ohio series of rocks where such shale covers occur, viz., at the surface of the Trenton limestone, where 800 to 1,000 feet of shales and intercalated beds of limestone of the Medina, Hudson river, and Utica epochs are found, at the surface of the Corniferous limestone, which is covered by 300 to 1,800 feet of the Ohio shale, and at the surface of the Berea grit, which is overlain by the best cover of the entire series, viz., the close-grained and nearly homogeneous Cuyahoga shale, 300 to 500 feet in thickness. Two of these, the first and the last, constitute the two main horizons of oil and gas in Ohio. The third is not notably productive thus far in Ohio, but it is the source of a small supply in other States.

The composition of an oil-producing series is thus seen to be essential to its functions. The order already pointed out cannot be departed from, but there must always be (1) an impervious cover; (2) a porous reservoir; and underneath the reservoir, the source is to be found.

STRUCTURE AS AFFECTING OIL AND GAS

ACCUMULATION.

But this order of arrangement is not enough in itself to insure any large concentration of oil or gas at any particular place. One other factor must be introduced, viz., structure. The strata which constitute the geological scale of the State nowhere lie, for any considerable extent, in horizontal planes. They are all more or less inclined. Sometimes they are bent into low folds or arches, and sometimes, though very rarely, there are abrupt descents and fractures. As a rule the dip, or angle of inclination to the horizon, of Ohio rocks is very small. It is better expressed as a fall of so many feet to the mile, than by angular measurements, which very seldom rise to one degree. Both the rate and the direction of the descent are uniform over large areas. The average dip for important portions of the State is between twenty and thirty feet; the direction depends, of course, upon the part of the State which is to be considered.

The movements of the strata here referred to have exerted a very important influence on the concentration of oil and gas in the reservoirs already described. If one of these sandstone strata, filled with salt-water, oil, and gas, and freely permeable laterally and horizontally for even miles at a time, were to be thrown into a system of low folds, what effect would this movement have upon the contents of the stratum? Would not a separation of gas, oil, and water be sure to follow, the gas finding its way to the summits of the arches, and the salt-water sinking to the bottoms of the troughs? Such a result would be inevitable under the conditions assumed.

The summits of the folds are called anti

clinals, and the troughs synclinals. The lines of direction of the anticlinals are called their axes. The influence of these facts of structure on gas and oil accumulation has been long recognized, or at least asserted, but there is not full agreement as to the part that it plays in the great fields among the geologists who have given most study to the subjects.

The facts that have come to light in the recent investigations of these subjects in Ohio seem to show the paramount influence of structure upon oil and gas accumulation. In the old fields, and in the new alike, irregularities of dip, involving change of direction, suspension, or unusual increase, have been found connected with the large production of both oil and gas in every instance where careful examination has been made. The composition of the series involved is identical for many thousand square miles, but so long as uniformity of dip is maintained, there is no valuable accumulation. As soon, however, as this uniformity is broken in upon, the valuable stocks of gas and oil come to light.

The "belt lines," in which the practical oil-well driller and operator of the main field puts so much confidence, so far as they stand for facts in nature, are probably structural lines. A map of the various centres of petroleum in the old field shows that they all extend in the northeasterly course which the main structural features of this part of the continent follow. The driller believes fortune to lie in the 45° or 22 line which leads out in a northeast or southwest direction from each centre of production. Experience justifies, to a certain extent, his confidence. The productive gas territory upon which Pittsburg now depends is limited to the summits of a few well-marked anticlinals, which all have a northeasterly trend. In regard to the latter, question can scarcely be raised. The predominant influence of structure is obvious. It seems probable that a careful enough system of measurements will show like lines of modified dip to traverse the great oil fields of Pennsylvania and New York.

The occurrence of gas and oil in almost all rocks that have a heavy shale cover would seem to result from exchanges affected by gravity. The oil is associated with saltwater in the stratum that contains it. There would be a constant tendency for the oil to reach a higher level at the expense of the water. It ascends through all the substance of the rock until it reaches the impervious roof, where it is gradually concentrated. the same principle, the separation of the gas from the oil is effected.

On

Some of the points that have been made under this head may be briefly restated, as follows:

1. Clay is largely connected with the primary accumulation of petroleum. The natural affinity that it has for substances of this class would lead to its combination with them wherever found. The great shale formation of Eastern Ohio, New York and Pennsyl

vania is the main source of the petroleum and gas of these regions. Clay does its work in this regard by reason of its chemical constitution.

2. As clay is the main agent in the primary accumulation of petroleum, sand takes a similar place in its secondary accumulation, or its concentration in valuable stocks. It does this by virtue of its physical character. A sandstone is a porous rock. Such sandstones as are found overlying or imbedded in the great shale formation are sure to become receptacles of oil.

3. Clay has another office in this connection to perform, and this office is dependent on its physical character. The sandstone stratum last described would become a receptacle of oil in any case, but if roofed with a sufficient thickness of clay shale by which its contents could be sealed and preserved, it would became a reservoir of oil or gas. All of the stocks of the old fields are held in sandstone or conglomerate reservoirs.

4. Limestone has been found, more clearly in Ohio, perhaps, than elsewhere, to replace sandstone in oil accumulation. All the phenomena of high-pressure stocks of oil and gas have recently been found in the Trenton limestone of Northern Ohio, but the presence and office of the shale cover are seen to be the same here as in the other fields. The term limestone in this connection is used with due care and precision. It is limestone, not "oil-sand" in the limestone, that contains Findlay gas and Lima oil. Pure magnesian limestone is the driller's "oil-sand" in these fields.

5. Widely diffused as are oil and gas in the paleozoic rocks of Ohio and adjacent States, so wide that the distribution of them may, without error, be styled universal, and widely extended as are the series of rocks that afford in their composition and relations the proper conditions for storage, it is still seen that their accumulation in profitable quantity depends on what might be called geological accidents. It is only or mainly along lines of structural disturbance that the great stocks are found.

THE ROCK PRESSURE OF GAS.

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 drillingtools, 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?

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 deeplyburied 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 princiciples 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 had 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 saltwater 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 oilfield, 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.

Certain obvious inferences follow the acceptance of this explanation:

pressure.

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

as

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 gas

holder 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.

SOURCES OF GAS AND OIL IN THE OHIO SCALE.

There are known at the present time four utilizable sources of gas and oil among the strata that underlie Ohio. They are as follows, named in descending order:

1. The Berea grit in Eastern Ohio.

2. The Ohio shale in Northern and Central Ohio.

3. The Clinton limestone in Sandusky, Wood, Hancock and Fairfield counties. 4. The Trenton limestone in Northwestern Ohio.

The Berea grit yields high-pressure gas and large stocks of oil under favorable circumstances, but these circumstances do not often recur. This stratum is doing but very little in supplying to the people of the State either gas or oil at the present time. Outside of Ohio in Western Pennsylvania it is found to be one of the most important repositories of this stored power that has been discovered in that highly favored territory.

The Ohio shale as a source of gas has already been briefly characterized in the account of this formation given on a previous page. It yields low-pressure gas in small amount at many places, but can never be made a source of large supply.

The two formations next to be named have special interest for us from the fact that their petroliferous character on the large scale was first demonstrated in Ohio. The first of them, indeed, has never been found to be an oil or gas rock elsewhere. It has not yet been proved to be a reservoir of any great value in Ohio, but moderate supplies of gas have been for some time derived from it in Fremont and in adjacent territory of Northern Ohio. In Lancaster, however, in Southern Ohio, the largest promise of the rock has recently been found. Wells drilled to the Clinton limestone, which is reached at a depth of 2,000 feet, have yielded as much as 1,000,000 cubic feet a day when first struck. The initial rock-pressure is high, viz., 700 pounds to the square inch. It is too early to draw safe conclusions as to the value of this discovery. All turns on the life of the wells. On account of their depth the drilling and casing are expensive. A well cannot be completed for less than $3,500 to $4,000. The facts at present in hand seem to betoken a short duration for the supply. A large amount of money is sure to be spent in the new field that the experience of Lancaster has brought to light.

It remains to describe in few words the remarkable discovery of gas and oil in the Trenton limestone that was made at Findlay in November, 1884.

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 limestone 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 oilrocks 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.

Surface indications of a sulphuretted and inflammable gas, escaping from the rocky floor of the village of Findlay, have been known since the country was first settled. The gas had, in fact, been utilized in a small way, viz., in lighting a single residence for more than forty years, but in 1884 the influence of Pittsburg had made itself felt through much of Ohio and drilling was begun here. At a depth of 1,100 feet a respectable flow of gas was secured. The success of this well was the first step in by far the most remarkable development that has ever taken place in the geology of Ohio.

It was more than a year before a great gas well was discovered in Findlay, but the Karg well, which was completed in January, 1886, fully deserves this name. Its daily yield when first opened was not less than 14,000,000 cubic feet.

The discovery of oil followed that of gas by a short interval, but the prolific character of the new rock was not established till the latter half of 1886.

The rapid extension of productive territory and its equally rapid limitations, the development of several distinct centres, as Bowling Green, Lima and St. Mary's, the great speculative excitement that broke out when the good fortune of the new gas-field began to be appreciated by manufacturers and investors, and the wonderful developments that have since taken place in the line of manufacturing industries, cannot be even touched upon in this connection. The salient points in the geology of the new fields are brought out in the summary that follows. The discovery comes from an unexpected quarter, viz., from the "black swamp of old time of Northwestern Ohio. Under its broad and level expanses a few hundred square miles have been found distributed through portions of five counties, within which are contained fountains of oil and reservoirs of gas of infinitely more value than any like accumulations hitherto discovered in the State, and fully deserving a place among the most

valued repositories of these substances in any quarter of the world.

The leading facts pertaining to the field can be summarized as follows:

1. In fourteen of the northwestern counties of Ohio (and like conditions prevail in contiguous territory in Indiana), the upper beds of the Trenton limestone, which lie from 1,000 to 2,000 feet below the surface, have a chemical composition different from that which generally characterizes this great stratum. They are here found as dolomite or magnesian limestone instead of being, as usual, true carbonate of lime. Their percentage of lime, in other words, ranges between 50 and 60 per cent. instead of between 80 and 90 per cent., as in the formation at large. These dolomites of Northwestern Ohio are mainly quite free from silicious impurities. The dolomitic composition seems to have resulted from an alteration of a true limestone. At least the occasional masses of true limestones charged with fossils, that are found on the horizon of and surrounded by the dolomite, are best explained on this supposition. In the change which has been endured, the fossils which the original limestones contained appear to have been for the most part discharged or rendered obscure, as is usual in this metamorphosis. crystalline character of the dolomite is often very marked, and there results from it a peculiarly open or porous structure. Its storage capacity is much greater than that of ordinary oil sandstones and conglomerates, so far at least as pores visible to the unaided eye are concerned. The change usually extends for ten to thirty feet below the surface of the formation. In some cases, however, sheets of porous dolomite are found as low as fifty feet and very rarely as low as 100 feet below the surface.

The

The area occupied by this dolomitic phase of the Trenton limestone in Ohio has already been indicated. The eastern and the southern boundaries pass through Lucas, Wood, Hancock, Allen, Auglaize and Mercer counties. It is possible that the line crosses some parts of Ottawa, Wyandot and Hardin counties.

There is good reason to believe that this phase extends far to the northward and westward, outside of the State limits to which it has here been traced. We know that the Trenton limestone is a dolomite when it pitches rapidly down from the northern boundary of Ohio to make the low-lying floor of the Michigan coal basin, and we also know that it is a dolomite when it rises from under that basin as a surface rock of the northern peninsula. In like manner it is a dolomite when it leaves the western boundary of the State under deep cover, and it is a dolomite when it reaches the surface once more in the Galena district of Illinois and Wisconsin.

South of the line laid down in Ohio there has not thus far been found a trace of the porous dolomite on which the oil of Lima and the gas of Findlay depend. The change is seen to be taking place in Shelby and