In spite of the enormous quantities of mineral oil that are
continuously drawn from the earth, and the many places from which it
may thus be drawn, geologists are still puzzled to account for it. If
it were commonly associated with coal the problem of its origin would
be solved at once. We should then be satisfied that natural mineral
oil is produced in the same manner as the artificial product, _i.e._,
by the heating and consequent distillation of certain kinds of coal or
of bituminous shales; but, as a matter of fact, it is but rarely that
petroleum is found in the midst of coal seams, though it is sometimes
so found.
I visited, some years ago, a coal-mine in Shropshire, known as “the
tarry pit,” thus named on account of the large quantity of crude
mineral oil of a rather coarse quality that exuded from the strata
pierced by the shaft. It ran down the sides of the shaft, filled the
“sumph” (_i.e._, the well at the bottom of the shaft in which the water
draining from the mine should accumulate for pumping), and annoyed the
colliers so seriously that they refused to work in the mine unless
the nuisance were abolished. It was abolished by “tubbing” the shaft
with an oil-proof lining built round that part from which the oil
issued. The “tar” as the crude oil was called, was then pumped out of
the sumph, and formed a pool which has since been filled up by the
_débris_ of the ordinary mine workings.
A publican in the Black Country of South Staffordshire discovered an
issue of inflammable vapor in his cellar, collected it by thrusting a
pipe into the ground, and used it for lighting and warming purposes, as
well as an attraction to customers.
These and other cases that might be cited, although exceptional, are of
some value in helping us to form a simple and rational theory of the
origin of this important natural product. They prove that mineral oil
_may_ be produced in connection with coal seams and apparently from the
coal itself. A sound theory of the origin of petroleum is of practical
as well as theoretical value, inasmuch as the very practical question
of the probable permanency of supply depends entirely on the nature of
the origin of that supply. Some very odd theories have been put forth,
especially in America.
Seeing that petroleum is commonly found associated with sandstone and
limestone, especially in cavities of the latter, it has been supposed
that these minerals somehow produce it. Turning back to the _Grocer_
for April 18, 1872, I find some speculations of this kind quoted from
the _Petroleum Monthly_. The writer sets aside altogether, as an
antiquated and exploded fallacy, the idea that petroleum is produced
from coal, and maintains “that petroleum is mainly produced from,
or generated through, limestone,” and argues that the generation of
petroleum by such rocks is a continuous process, from the fact that
exhausted wells have recovered after being abandoned, his explanation
being “that the formerly abandoned territory was given up because the
machinery for extracting petroleum from the earth exceeded in its
power of exhausting the fluid the generative powers by which it is
produced;” these generative powers somehow residing in the limestone
and sandstone, but how is not specified.
Some writers have, however, gone a little further toward answering the
question of how limestone may generate petroleum. They have pointed
to the fossilized remains of animals, their shells, etc., existing
in the limestone, and have supposed that the animal matter has been
distilled, and has thus formed the oil.
If such a process could be imitated artificially by distilling some of
the later deposits of similar fossil character this theory would have a
better basis, or even if a collection of oysters, mussels, or any other
animal matters could by distillation be shown to produce an oil similar
to petroleum.
The contrary is the case. We may obtain oil from such material, but it
is utterly different from any kind of mineral oil, while, on the other
hand, by distilling natural bituminous shales, or cannel coal, or peat,
we obtain a crude oil almost identical with natural petroleum, and the
little difference between the two is perfectly accounted for by the
greater rapidity of our methods of distillation as compared with the
slow natural process. We may go on approximating more and more nearly
to the natural petroleum by distilling more and more slowly. As it
is, the refined products of the natural and artificial oil which is
commercially distilled in Scotland, are scarcely distinguishable—some
of them are not at all distinguishable—the solid paraffin, for
example. I now offer my own theory of the origin of oil springs.
To render this the more intelligible, let us first consider the
origin of ordinary water springs. St. Winifred’s Well, at Holywell,
in Flintshire, maybe taken as an example, not merely on account of
its magnitude, but because it is quite typical, and is connected with
limestone and sandstone in about the same manner as are the petroleum
wells of Pennsylvania.
Here we have a wondrous uprush of water just between the sandstone
and mountain limestone rocks, which amounts to above twenty tons per
minute, and flows down to the Dee, a small river turning several
water-mills. It is certain that all this water is not generated either
by the limestone or the sandstone from which it issues, nor can it be
all “generated” on the spot. The true explanation of its origin is
simple enough.
The mountain limestone underlies the coal measures and crops up
obliquely at Holywell; against this oblique subterranean wall of
compact rock impermeable to water, abuts a great face of down-sloping
strata of porous sandstone and porous shales. These porous rocks
receive the rain which falls on the slopes of the Hope Mountain and
other hills which they form; this water sinks into the millstone grit
of these hills and percolates downwards until it reaches the limestone
barrier, into which it cannot penetrate.
It here accumulates as a subterranean reservoir which finds an outlet
at a convenient natural fissure, and, as the percolation is continuous,
the spring is a constant one. Some of the water travels many miles
underground before it thus escapes. Hundreds of other smaller instances
might be quoted, the above being the common history of springs which
start up whenever the underground waters that flow through porous
rocks or soil meet with compact rocks or impermeable clay, and thus,
being able to proceed no further downwards, accumulate and produce an
overflow which we call a “spring.”
If water can thus travel underground, why not oil?
Although the oil springs or oil wells are not immediately above or
below coal seams, they are all within “measurable distance” of great
coal formations—the oil territory of Pennsylvania is, in fact,
surrounded by coal, some of it anthracite, which is really a coke, such
as would be produced if we artificially distilled the hydrocarbons from
coal, and then compressed the residue, as the anthracite has certainly
been pressed by the strata resting upon it.
The rocks in immediate contact and proximity to coal seams—“the coal
measures,” as they are called—are mostly porous, some of them very
porous, and thus if at any period of the earth’s long history a seam
of coal became heated, as we know so many strata are, and have been
heated, a mineral oil would certainly be formed, would first permeate
the porous rocks as vapor, then be condensed and make its way through
them, following their “dip” or inclination until it reached a barrier
such as the limestone forms.
It would thus in after-ages be found, not among the coal where it was
formed, but at the limestone or other impermeable rock by which its
further percolation was arrested.
This is just where it actually is found.
Limestone, although not porous like shales and sandstones, is specially
well adapted for storing large subterranean accumulations, on account
of the great cavities to which it is liable. Nearly all the caverns in
this country, in Ireland where they abound, in America, and other parts
of the world, are in limestone rocks; they are especially abundant
in the “carboniferous limestone” which underlies the coal measures,
and this is explained by the fact that limestone may be dissolved by
rain-water that has oozed through vegetable soil or has soaked fallen
leaves or other vegetable matter, and thereby become saturated with
carbonic acid.
Where the petroleum finds a crevice leading to such cavities it must
creep through it and fill the space, thereby forming one of the
underground reservoirs supplying those pumping wells that have yielded
such abundance for a while and then become dry. But if this theory is
correct it does not follow that the drying of such a well proves a
final stoppage of the supply, for if the cavity and crevice are left,
more oil may ooze into the crevice and flow into the cavity, and this
may continue again and again throughout the whole oil district so long
as the surrounding feeders of permeable strata continue saturated, or
nearly so. The magnitude of these feeding grounds may far exceed that
of the district wherein the springs occur, or where profitable wells
may be sunk, seeing that the localizing of profitable supply depends
mainly on the stoppage of further oozing away by the action of the
impermeable barrier.
A well sunk into the oozing strata itself would receive a very small
quantity, only that which, in the course of its passage came upon
the well sides, while at the junction between the permeable and the
impermeable rocks the accumulation may include all that reached the
whole surface of such junction or contact—many square miles.
To test this theory thoroughly it would be necessary to make borings,
not merely at the wells, but in their neighborhood, where the porous
rocks dip towards the limestone, and to bring up sample cores of these
porous rocks, and carefully examine them. Dr. Sterry Hunt has done this
in the oil-yielding limestone rocks of Chicago, but not in those of the
nearest coal-measures.
As the oil industry of America is of such great national importance, an
investigation of this kind is worthy of the energies of the American
Government geologists. It would throw much light on the whole subject,
and supply data from which the probable duration of the oil supply
might be approximately calculated.
Such an investigation might even do more than this. By proving the
geological conditions upon which depend the production of petroleum
springs, new sources may be discovered, just as new coal-seams have
been discovered, in accordance with geological prediction, or as the
practical discovery of the Austrian gold-fields was so long preceded
by Sir Roderick Murchison’s theoretical announcement of their probable
existence.
When the “kerosene wells” were first struck, the speculations
concerning their probable permanency were wild and various. Some
maintained that it was but a spurt, a freak of nature limited to a
narrow locality, and would soon be over; others asserted forthwith that
American oil, like everything else American, was boundless. Neither
had any grounds for their assertions, and therefore made them with the
usual boldness of mere dogmatism.
Then came a period of scare, started by the fact that wells which at
first spouted an inflammable mixture of oil and vapor high into the
air soon became quiescent, and from “spouting wells” became “flowing
wells,” merely pouring out on the surface a small stream at first,
which gradually declined to a dribble, and finally ceased to flow at
all. Even those that started modestly as flowing wells did the latter,
and thus appeared to become exhausted.
This exhaustion, however, was only apparent, as was proved by the
application of pumps, which drew up from wells, that had ceased either
to spout or flow, large and apparently undiminishing quantities of
crude oil.
Further observation and thought revealed the cause of these changes.
It became understood that the spouting was due to the tapping of a
rock-cavity containing oil of such varying densities and volatility
that some of it flew out as a vapor, or boiled at the mean temperature
of the air of the country or that of the surrounding rocks. Such being
the case, the cavity was filled with high-pressure oil-vapor straining
to escape. If the bore-hole tapped the crown or highest curve of the
roof of such an oil-cavern, it opened directly into the vapor there
accumulated, and the vapor itself rushed out with such force that a
pillar of fire was raised in the air if a light came within some yards
of the orifice. We are told of heavy iron boring-rods that were shot up
to wondrous heights—and we may believe these stories if we please.
If the bore-hole struck lower down, somewhere on the sloping sides or
in the shallow lower branches of the oil-cavern, it dipped at once into
liquid oil, and this oil, being pressed by the elastic vapor of the
upper part, was forced up as a jet of spouting oil.
In either case these violent proceedings soon came to an end, for as
the vapor or oil poured out, the space above the oil-level where the
vapor had been confined was increased, and its pressure diminished,
till at last it barely sufficed to raise the oil to the surface, and
afterwards failed to do that.
It is quite clear from this that the supplies are not “inexhaustible.”
The quantity of vapor having been limited, there must also be a
limit to the quantity of oil giving off this vapor; the space in the
oil-cavern occupied by this vapor having been limited, there must be
a limit to the space occupied by the oil. The quantity of oil may be
ten times, a hundred times, a thousand times, or ten thousand times,
greater than that of the vapor, but in either or any case it must come
to an end at last, sooner later.
If there were but a few wells here and there, as at other similar
places, such as Rangoon, the Persian oil-wells, etc., the pumping
might continue for centuries and centuries; but this is not the case
in America. The final boundaries of the oil-bearing strata may not
yet have been reached; but so far as they are known they are riddled
through and through, and pumped in every direction, so that the end
must come at last, though with our present knowledge we cannot say
_when_.
We can, however, say _how_ it must come. It will not be a sudden
stoppage, but a gradual exhaustion indicated by progressive diminution
of supply. We shall not be suddenly deprived of this important source
of light and cheerfulness; but we may at any time begin to feel the
pinch of scarcity and consequent rise of price. This rise of price will
check the demand, and bring forth other supplies from sources that now
cannot be profitably worked on account of the cheapness of American
petroleum.
Many of the countries now largely supplied from America have
oil-springs of their own, which a rise of price will speedily bring
into paying operation.
We have nothing to fear. The fact that in spite of the ruinous prices
that have recently prevailed the Scotch oil-makers continue to exist
at all, shows us what they may do with a rise of even a few pence per
gallon. The thickness and area of the dark shales from which their oil
is distilled are so great that their exhaustion is very far remote
indeed. The Americans have similar shales to fall back upon when the
spontaneous product ceases to flow, but they are quite incapable of
competing with us at home on equal terms—that is, when both have to
obtain the oil as a manufactured product of artificial distillation.
If anything like moderation were possible in America, the first
indications of scarcity would be followed by some economy in working;
but this is not to be anticipated. It is more likely that the first
rise of prices will attract additional speculation, and the sinking
of more wells in the hope of large profits, and this of course will
shorten the period of gradual exhaustion, the commencement of which
may, for aught we know, be very near at hand, especially if the new
projects for using petroleum as furnace fuel under steam boilers, and
for the smelting, puddling, and founding of iron and other metals, are
carried out as they may be so easily at present prices, and with the
aid of pipe-lines to carry the crude or refined oil from the wells to
any part of the great American continent where it may be required in
large quantities.
The old story of the goose that laid the golden eggs seems to be in
course of repetition in Transatlantic Petrolia.
* * * * *
Since the above was written I have received from Dr. Sterry Hunt a
copy of his interesting “Chemical and Geological Essays,” in one of
which he expounds a theory of the origin of petroleum. He states that
it appears to him “that the petroleum, or rather the materials from
which it has been formed, existed in the limestone rocks from the time
of their first deposition,” and “that petroleum and similar bitumens
have resulted from a peculiar transformation of vegetable matters, or
in some cases of animal tissues analogous to these in composition.”
The objections on page 275 apply to the animal tissues of this theory,
and as regards the vegetable matter I think it fails from the want of
anything like an adequate supply in these limestone rocks.