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and Bellaire are represented. The series gains somewhat in volume also to the south ward, but in the first included sections east of the limestone belt in northern Ohio, it is not more than 600 to 800 feet thick. What the volume of the shales originally was in northwestern Ohio, it is impossible to say. We do not now find more than 200 feet of the shales in any of the sections that remain, but it must be borne in mind that this quarter of Ohio has been especially exposed to glacial denudation, and that this formation in particular has little resisting power. The enormous quantity of the shale that is found in the bowlder clay that covers the western half of the state in particular, illustrates in a striking manner the great amount of waste which the shales have suffered from this agency.

From the eastern border of the limestones the shales thicken very rapidly to the east and southeast. This is represented in the section from Elyria to Akron which appears in the preceeding chapter, page 338.

The limestone is seen to descend to the southeast, while the Berea grit inclines in the opposite direction.

The interval between the Berea grit and the Upper Helderberg limestone at Elyria is 800 feet, while at Akron the same interval is 1,862 feet. This important feature in the structure of this part of the state has already been discussed in Chapter I, under the head of the Akron axis, page 57, but the fact to be noted here is the rapid increase of the shales in a southeasterly direction. The interval between the horizons named above at other points in the state are given below. The points along the western margin where the measurements are smallest are named in the first column, but even here there are some localities as Mt. Vernon, that are far removed from the margin of the formation.

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From central Ohio eastward the drill has never gone deep enough to find the bottom of the Ohio shales. In Washington county, as has been shown in the record of the Rake well, at 1,259 feet below the Macksburg sand, the drill was still in unbroken shale. It is probable we need to go down at least 1,200 feet further to find the limestone floor.

The limestone has been reached at various points on the shore of Lake Erie, among which may be named Elyria, Cleveland, Erie and North East, Pennsylvania, and Fredonia, New York. It rises to the surface again near Buffalo.

The thickness of the shale is seen, from the statements here given, to be altogether a question of geography. On the western border of the main area it ranges from 250 to 650 feet in thickness, while to the east and south it strengthens, rapidly reaching a total of 1,200, 1,800, 2,400, and even 2,600 feet, and in the two last cases, which are in the Ohio Valley, the series is still unexhausted.

The composition of the series, so far as the alternating bands of black, gray, blue and greenish-blue shales are concerned, has already been described in Chapter I, and it will be much more fully illustrated in the well records to be given in the various parts of the field.

SOURCE OF SHALE, OIL AND GAS.

The shales in themselves considered are but small producers of oil. Not a single shale well is known, the supply of which is derived exclusively from the shale, that has ever produced more than ten or fitteen barrels of oil in a day, and even such yields have been limited to very few days at most. Where interrupted by sandstones, however, or even by sandy shales that are buried in them, all may be changed. Such sandstones are likely to become oil-sands, and to constitute the great reservoirs from which almost fabulous amounts of oil and gas flow forth when these reservoirs are reached by the drill. Although no great accumulations of oil are found in the shales proper, it would be wrong to infer that they are poor in petroleum. On the contrary, they contain much more than any other of the strata with which they are associated, the great sandstone reservoirs not excepted, but it is in a distributed condition that the petroleum occurs. The entire body of the shale carries a determinable percentage. The percentage is small, but the aggregate is vastly larger to the square mile than any square mile of the most prolific oil-field has ever yielded to the drill. Professor N. W. Lord, Chemist of the Survey, determined the amount of petroleum existing as such in normal samples of Ohio black shale. He found the amount to be slightly less than one-fifth of one per cent., but he learned from his investigations that the finer the division of the shale, the larger was the percentage of oil obtained. It was therefore certain that the true percentage exceeded that reported; further than this, there had, undoubtedly, been some loss of petroleum from the exposure of the shale, but even on the percentage obtained, if the shale series is counted at the low average thickness of 1,000 feet for its entire area, the total amount per square mile passes far beyond the limits of clear comprehension.

Calculations show that it would exceed ten million barrels (10,000,000) to the square mile, and such figures stand, of course, merely for numbers that turn up in arithmetical processes, but to which we are unable to attach any definite significance further than to recognize in them very large numbers.

The oil-stock of the shales is large, as thus appears, but the oil-production is very small. The case, however, stands somewhat better with regard to gas. Here again the lack of sandstone reservoirs forbids large accumulations, but the shales themselves yield gas in such volume as to become of considerable economic importance. There can be no question but that gas is derived directly from the oil of the shales. Deep as the drill goes down the shales are found charged to the extent noted above with petroleum, which begins to break up at once into volatile compounds when exposed to the air.

SURFACE INDICATIONS.

Both oil and gas, as thus described, are found rising slowly through the shales to the surface at multitudes of the outcrops of this formation. No doubt most of the outcrops have been thus distinguished at some time in their history, but at many points the accessible stocks have been so reduced as to be no longer conspicuous or even noticeable. The movement of the oil to the surface is probably the result of a slow circulation in the shales, gravity bringing the oil upward as water descends. When the oil and gas appear in this way, the oil as a thin film over the water that escapes, and the gas issuing from the seams and joints of the shales, sometimes in bubbles through the water, and sometimes in small but steady streams, they are called surface indications, and great account is made of them in the search for oil and gas.

At many points such indications have been found to be the forerunners of the largest production of oil and gas. The Findlay field, for example, always abounded in these evidences of a great stock of gas buried beneath the surface, but it was a long while before any one was prepared to interpret these indications in the ordinary way.

The surface indications of the Ohio shale stand for no large accumulations, but they are constantly interpreted, or rather misinterpreted, as if they did. Such escape as is here described is the normal condition of the shales. It testifies to the distributed stocks of petroleum that lie below and that are absolutely universal. They are found wherever the shales are found. The main outcrops of the shales are not less than 400 miles

in length in Ohio, and there are but few miles of these outcrops except where drift deposits inter ere, in which oil films and gas bubbles cannot be found. Since natural gas has begun to attract such general attention these surface indications are constantly being rediscovered, and a week does not pass without the published announcement of the sure presence of abundant stocks of gas along these shale outcrops, amply sufficient for entire districts of the state, but when the drill descends at the most promising of these points it releases but a weak supply, adapted in its best condition only for domestic use on a very small scale.

Several hundred wells have already been drilled in various parts of the state in the shale formation, and the conclusions drawn from so large a number of examples are not at all likely to be set aside by any experience that is yet to be obtained. We know both the quality and the quantity of the bituminous products that the shales are able to supply.

CHARACTER OF SHALE GA3.

The gas yielded by the Ohio shale is definitely characterized. It is low-pressure gas-in other words, it does not attain the high-pressure of the great wells which are used for large public supply. This is the same as saying that it never produces a large daily volume of gas; but terms like these are relative. What is high-pressure, and what is a large daily production of gas? The Findlay wells reach a closed pressure of a little less than 400 pounds per square inch. The great wells of Pennsylvania show pressure much in excess of the Findiay wells, reaching, according to good authority, a pressure of at least 650 pounds per square inch. Pressures much greater than this are reported.

In contrast with these fields the closed pressure of shale gas-wells has rarely been known to rise beyond 100 pounds to the square inch. In almost all the cas's noted, the pressure has remained much below 100 pounds. What are called good wells show a maximum of fifty, sixty, or seventy pounds.

Again, the maximum production of the wells of the Findlay gasfield is 15,000,000 cubic feet per day. Wells are not counted of much force or value, the daily output of which does not exceed one-half million cubic feet. The great gas-wells of western Pennsylvania exceed the largest production of the Ohio field by several million cubic feet.

In contrast with this no shale gas-wells have been measured that produced more than 100,000 cubic feet per day, and it is very rare for them to reach this figure. They are counted excellent wells when they reach 50,000 cubic feet per day. What is meant by relatively low-pressure and small production in shale gas-wells, appears from the facts and comparisons here given.

The composition of the shale gas proper has not been determined by the Survey, but it is doubtless identical with the gas found in the Berea grit. An analysis of the latter, from the Neff wells, made several years since by Professor E. W. Morley, of Western Reserve University, is given below:

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The analyses recently made of natural gas in the Pennsylvania fields, and published on page 400, can be compared with these results. They will be found to agree fairly well. The marsh gas and ethyl hydride reported here can be counted together as paraffins and the total amount is seen to be 93.60 per cent.

The stocks of gas derived from the shale, although moderate or small in amount, are quite persistent, and therefore valuable. No gas supply is known that is more enduring than the supply derived from the shales. Natural gas of this description rises, in many cases, with the water of springs, and from all that we can see, it is as persistent as the water which it accompanies. The gas springs of Fredonia, New York, have been known since the country was first occupied, and the gas produced by them has been utilized in the town for lighting purposes since 1821. Wells have been drilled by the side of the original springs, and have interfered with the latter to some extent, but the flow still continues from the old sources. From wells drilled into the shales expressly for gas to be utilized as light, heat, or power, we cannot obtain any decisive testimony as to their durability, from the fact that they are all of comparatively recent date, but so far as it goes, the experience derived from them is quite satisfactory and assuring. Wells drilled fifty years ago are still fairly productive. In some cases they have been furnishing a steady supply since they were drilled, without requiring a dollar of outlay during the entire interval. The production is probably gradually diminishing in most cases, but enough is generally left to enable the wells to do all that is expected of them.

The two facts named above, namely, the low-pressure of the shale gas, and its persistency, point out in very clear light its special adapta

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