the Nágá territory.1 Mr. R. A. Townsend has reported officially upon the Assam oil fields.2 The most promising sites in Assam appear to lie within the concessions obtained by the Assam Railways and Trading Company, which has for some time carried on drilling · operations at Digboi and Makum, the results of which will be found recorded in the next section.

The Punjab deposits were described by Mr. A. Fleming in 18483 and 1853,4 by Mr. Maclagan in 1862,5 and by Mr A. Fenner in 1866.6 Mr B. S. Lyman has more fully described them. He states that some of the natural springs yield "as much as 3 quarts per day," and a boring at Gunda is said to have yielded at 75 feet about 11 gallons daily for six months.

An oil field, of which a description is given in the following section, also appears to exist in the Khátan Valley, in Baluchistan.

1 Mallet, "On the Coal Fields of the Nágá Hills, bordering the Lakhimpur and Sibsagar districts, Assam," Mem. Geol. Survey of India, xii. part 2.

2 Report on the Assam Petroleum Deposits. Simla, 1888.

3 Journ. Asiatic Soc. Bengal, xvii.

5 Suppl. to the Punjab Government Gazette.

Ibid., xxii.

6 Proc. Punjab Government Public Works Department.

7 Rep. on the Punjab Oil Lands. Government Press, Lahore.

32

SECTION II.

THE GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION OF PETROLEUM AND NATURAL GAS.

IN the solid, liquid, and gaseous forms, bituinen is one of the most widely-distributed of substances. It is found in greater or lesser quantity in almost every part of the globe, and its geological limits. include the whole range of strata, from the Laurentian rocks to the newer members of the Tertiary period.

As early as 1822, Knox1 detected the presence of "bitumen" in a large and varied series of rocks and minerals, including greenstone from County Down and Newry, basalt from the Giant's Causeway and Disco Island, hornblende from Schneeberg, augite from Arendal (Norway), and felspar from Killiney and Aberdeen. Among the few rocks in which he failed to discover this substance, Carrara marble deserves particular mention.

5

Silvestri 2 found a crystallisable paraffin in a basaltic lava at Paterno, near Mount Etna, and in the lava from that mountain. The occurrence of bitumens has been noticed in Laurentian rocks at Nullaberg,3 the melaphyre of Scotland, of Bohemia, and of Oberstein in the Palatinate, the granite of Cornwall and Kiel, the trap of Newhaven, Conn., the greenstone trachyte of Hungary, and many other of the older and metamorphic rocks. Oil and gas are, in fact, almost universally distributed through unaltered, and but little altered, strata of all ages, although their presence is less marked in the oldest deposits and in the more recent and superficial deposits which have become more broken and metamorphosed than in the deeper-seated

strata.

Notwithstanding this wide dispersion, the deposits which are worked commercially are mainly comprised within the Silurian and Carboniferous epochs in the Western Hemisphere, and the Eocene and Miocene deposits of the Tertiary period in the Eastern Hemisphere.

Pressure in Wells.-The oil and gas frequently issue from the wells at great pressure, more especially when first liberated. This pressure is sometimes so great that, when the oil stratum is reached, the boring tools are expelled from the borehole, and the oil escapes in a fountain rising high above the derrick, and frequently resisting all attempts to control its flow. Such "spouting" wells or "fountains" of

Phil. Trans., 1822, 313, and 1823, 517.

3 Ingelström, Geol. Mag., iv. 160.

5 Höfer, Das Erdöl, 1888, 74.

4

2 Gaz. Chim. Ital., xii. 9.

Q. J. Geol. Soc., xi. 468.

Höfer, loc. cit.

oil have been particularly frequent in Russia, and have often resulted in great waste, and even in ruin to the owners of the well, as described in the previous Section.

In the gas wells of the United States, the gas pressure has in many cases been very carefully measured. It may be ascertained either by placing a pressure-gauge in the freely-escaping gas as it issues from the casing, in which case the pressure is described as "open pressure," or by measuring the actual pressure in the closed well, when it is termed the "closed pressure.' The latter method, which measures the true rock pressure, is the only one of value, as it alone yields data which assist researches into the conditions under which the oil and gas are contained in the earth.

The highest pressures occur in the deepest wells. The closed pressure in the Trenton limestone in Ohio and Indiana is about 200-300 lbs. per square inch, although a much higher pressure has been registered in many wells, notably the "Loomis and Nyman well at Tiffin, which showed over 600 lbs., and the "Pioneer" well at Findlay, which yielded its gas at 450 lbs. closed pressure. The gas wells of Pennsylvania indicate about double the pressure of those drilled in the Trenton limestone, 600-800 lbs. not being unusual, and even 1,000 lbs. having been recorded.

Cause of the Pressure.-Three theories have been propounded as to the cause of this pressure:

1. That it results from the weight of the overlying strata.

2. That it is due to water pressure, as in artesian wells, the percolating water which enters the stratum at its outcrop forming the "head."

3. That it is caused by the gradually accumulating gas having had no opportunity for escaping, and being thus brought into a highly compressed condition.

The first theory is evidently untenable, and is now practically abandoned. Even the most friable of the reservoir rocks is capable of resisting the pressure of the overlying deposits; thus, the weakest portions of the Trenton limestone have been shown to withstand a crushing weight of 720 tons to the square foot, whereas the pressure on the stratum at the bottom of a well of over 1,000 feet in depth would only be about 80 tons. The rocks are also found perfectly compact at all depths reached by the drill.

The second theory, generally known as the "hydrostatic" or "artesian" theory, has many distinguished advocates, and has been particularly upheld by those who have closely studied the great gas fields of Ohio and Indiana; while, on the other hand, it appears to be quite inapplicable to the fields of Pennsylvania and New York, where, if not in all other fields, the third theory is evidently perfectly tenable. It was brought into prominence by Professor J. Ĉ. White in 1887, and is strongly emphasised in the reports of Dr. Orton on the oil and gas fields of Ohio and of Kentucky.

1 See the remarks of Professor Lesley in the Annual Rep. of the Penn. Survey, 1885.

As a general rule, the water in the wells drilled into the Trenton limestone in Ohio and Indiana rises to about 600 feet above sea level, when the gas and oil become exhausted. Thus, in a well at Lindsay, Sandusky Co., Ohio, the water rose to 600 feet above tide level when the well became "dry" while at Huntingdon and other districts in the Wabash Valley, a rise of 615 feet above tide level has been observed. These observations coincide with the outcrop of the Trenton limestone on the shores of Lakes Huron and Superior, at about 600 feet above tide level.

Dr. Orton has calculated the pressure which should be found in wells at certain depths, if produced by a head of water equal to the depth of the well below tide level, and has found a remarkable agreement with the actually-measured pressures in many wells of Ohio. He considers that his results distinctly prove that "the rock pressure of Trenton limestone gas is due to a salt-water column measured from about 600 feet below tide, to the level of the stratum which holds the gas."

A notable exception to this general rule is furnished by the "Simon's" well in Wood Co., Ohio, where a pressure of 520 lbs. has been noticed, a greater pressure than has been observed in any other part of the field. It corresponds to a head of 1,090 feet of water and, apparently, cannot be accounted for on the artesian theory. According to Dr. Orton 2 the great rock pressures in Pennsylvanian gas wells may be also explained by the artesian theory.

Professor W. J. M'Gee3 explains the pressure of the gas of Indiana on this hypothesis as follows:

"The cause of this enormous pressure is readily seen in Indiana. The Cincinnati arch (in which the gas of the great Indiana field is accumulated), is substantially a dome about 50 miles across, rising in the centre of a stratigraphic basin fully 500 miles in average diameter. Throughout this immense basin, the waters falling on the surface are, in part, absorbed into the rocks and conveyed towards its centre, where a strong artesian flow of water would prevail were the difference in altitude greater; and the light hydrocarbons floating upon the surface of this ground water are driven into the dome, and there subjected to hydrostatic pressure equal to the weight of a column of water whose height is the difference in altitude between the water surface within the dome and the land surface of the catchment area about the rim of the enclosing basin. Accordingly, the static pressure is independent of the absolute altitude of the gas-rock and of its depth beneath the surface, except in so far as these are involved in the relative altitudes of the gas-rock and a catchment area, perhaps scores or even hundreds of miles distant. Gas pressure and oil pressure may, therefore, be estimated in any given case as readily and reliably as artesian water pressure; but whilst the water pressure is measured approximately by the difference in altitude between the catchment 1 First Annual Rep. of the Geol. Survey of Ohio, 1890, 102.

2 Geol. Survey of Kentucky. Report on the occurrence of Petroleum, &c., in Western Kentucky, 1891. 3 The Forum, July, 1889.

area and well-head, that of gas is measured approximately by the difference in altitude between catchment area and gas-rock, and that of oil is measured by the same difference, minus the weight of a column of oil equal to the depth of the well. It follows that the static pressure of gas (as indicated at the surface) is always greater than that of oil, particularly in deep wells. It follows, also, that the pressure, whether of gas or oil, is not only constant throughout each field, but diminishes but slightly, if at all, on the tapping of the reservoir, until the supply is exhausted; and hence that pressure is no indication of either abundance or permanence of supply."

The artesian theory is somewhat supported by the fact that water rises in the wells of Ohio and Indiana as they become exhausted, for, says Dr. Orton, the cause which raises the oil and gas must be the same which operates upon the water, and can be only of artesian origin. It is, however, strongly opposed by Professor J. P. Lesley, Mr. Carll, and others. In his letter of transmissal to Carll's Seventh Report on the Oil and Gas Wells of Western Pennsylvania, 1890, Lesley states his belief that the hydraulic pressure head at the Trenton outcrop in Canada could never account for the well pressure one or two hundred miles to the south, and adds,—“Considering all the local underground obstacles to free flow, I cannot think that even the highest gas pressure should maintain itself against rock friction for even so short a distance as 20 miles."

In that report (p. 13) Mr. Carll remarks that he has never witnessed or heard of a single circumstance to support the artesian hypothesis, and states that "the theory, as applied to Pennsylvanian oil and gas wells is delusive and untenable, and the cause of the great rock pressure witnessed must, therefore, be sought for in some other direction." He further points out that if oil wells are subject to hydrostatic pressure the flow should be constant while it lasts, and dependent on the speed with which the water enters the oil-rock reservoir. After displacing the oil, water ought to follow. Yet it is found that the output decreases gradually, and sometimes rapidly, and the wells may be pumped for years, even with gas pumps, without being flooded by water. Gas pumps have been used in the Triumph district since 1869, and the exhaustion is such that a vacuum gauge at any of the wells registers a vacuum of 12 to 13 lbs. per square inch.

As regards the low-pressure gas from the Ohio and other shales, Dr. Orton1 observes that the artesian theory does not apply.

Oil and Gas Reservoir-Rocks.-Most of the porous oil and gas rocks, particularly the sandstones and conglomerates, may be regarded as merely reservoirs whose contents have been generated from the underlying strata, frequently fossiliferous and highly compacted shales, or from some other source; but in the case of many others, more especially certain limestones, the oil and gas have usually been regarded as indigenous to the containing strata. This belief as to the origin of the oil in many of the strata in which it is now found is almost universally accepted. From observations made near Buffalo 1 First Annual Rep. of the Geol. Survey of Ohio, 249.