CHAPTER V.

OCCURRENCE OF PETROLEUM.

Primary and Secondary Deposits.

A GREATER natural accumulation of petroleum in the crust of the earth or on its surface is called a deposit, and may be either primary or original, or secondary or shifted.

Petroleum, as will be shown later on, is of organic origin, and hence could only be formed where organisms were present. For this reason it cannot have originated in the massive or eruptive rocks, just as little as in the oldest azoic (archæan) strata. The seat of the formation of petroleum can, therefore, be located only in strata more recent than the archæan, which also agrees with all assured facts known at the present time.

The primary or original deposits of petroleum may, therefore, belong to the strata of the Silurian age up to those of the present time. It is a remarkable fact that they are wanting, or are of no technical importance, in the strata carrying larger coal

measures.

In the primary deposit the organisms accumulated, and conditions prevailed which promoted the process of transformation and permitted an accumulation of the product of transformation -the petroleum. It could, therefore, be only a sedimentary deposit. Besides the organisms, inorganic masses, such as debris, sand, mud, or the hard portions of animals, were separated, so that the oil formed later on exuded through this collection of strata, which in time might also have hardened and been partially transformed. The primary deposit is, therefore, an impregnated stratum or deposit, according to whether it retains the plate-form or the latter passes over into the lenticular form. Both types, which differ geometrically only, are found

in nature.

Sand, debris, coarse-grained sandstone, conglomerate, especially when poor in cementing material, porous limestone, and, in general, rock with many and large pores or many cracks and fissures, will absorb the oil in greater quantities and, when opened, yield it more rapidly than compact varieties of rock.

H. Römer some time ago showed the presence of up to 17 per cent. of lime in the oil-bearing sandstone of Oedesse (Hanover). Br. Walter also points out that the oil-bearing Ropianka sandstones are rich in lime, and supposes that the number of pores is increased by the lixiviation of calcium carbonate. These strata are in fact rich in calcite veins. These indications deserve further consideration, since they may possibly be of advantage for the practice.

J. F. Carll's investigations prove that the oil-sand of Pennsylvania contains of its bulk of crude oil, and he supposes that under pressure this capacity may increase to 1.

The occurrence of oil in the Carpathian menilite schists must be considered an original deposit, as well as a great portion of the North German finds in the Jura and Wealden strata (according to H. Credner).

In these original deposits the oil frequently stands under a higher pressure, especially in consequence of included gases formed during the processes of transformation, so that in drilling the heavy string of tools may be ejected from a deep bore-hole. In consequence of this internal pressure the oil endeavors to escape in all directions. If the vicinity of the original deposit consists of porous rock, the oil will penetrate the latter, and the size of the deposit will thus be increased. By this process it may, however, happen that the oil is distributed over such large masses of rock that its flow in a shaft or bore-hole decreases to such an extent as to render the working unprofitable. The gases penetrate the rock with more ease and for a greater distance, and in nearly every oil region are therefore considered a good indication of the speedy opening of an oil deposit.

By the above-mentioned wandering or migration the oil is afforded more chances to evaporate, so that in consequence of

these processes a formerly productive deposit may become simply bituminous rock. The formation of the latter, however, may also have been due to the fact that but few organisms capable of forming oil were deposited in it, or that the processes on which transformation depends were not favorable. This explains partially why the same group of strata may be only locally oil-bearing.

The processes by which the original extent of a primary deposit of petroleum may be enlarged have been mentioned above; in such a case it will frequently be impossible to determine the original limits of the deposit. However, the enlarged, as well as the original, deposits must show the type of a sedimentary deposit, the surrounding porous rocks being also deposited in strata.

However, the pores of solid rocks alone do not afford much opportunity for migration to the oil, and the less so the smaller they are, this view being justified by experiments with rocks into which water was forced by means of high pressure. If we take into consideration that the pores of rocks are frequently closed by the specifically heavier water-the so-called rock-moisture-and further that the entering oil experiences considerable loss of pressure in consequence of capillarity and friction, which have to be overcome, an enormously high pressure would have to be supposed to make the further migration of the oil in the pores of the rock possible.

The conditions are, however, more favorable in loose masses, sand, debris, etc., the interspaces being larger than in the more compact rocks, and therefore readily penetrable.

The escape of the oil standing under internal pressure from its primary deposit through fissures communicating with the latter is very possible, and the more so the wider these fissures are and the greater their number. The oil enters these fissures, fills them, and according to the pressure, ascends frequently to the surface, forming there an oil well. This occurrence in fissures, as, for instance in some localities in Ohio and West Virginia, constitutes a secondary or shifted deposit; it fills

size and direcshort, their geofrequent changes

She deh and . therefore, a structure corresponding to veins *tva. The reins frequently change Now they divide and site again, and merci proportions are subject to more than those of the beds (primary of deposits) which lie conontact to the surrounding strata. For this reason turns, bendings and similar disturbances of the c bed may be recognized from those of the adjoining strata running in an analogous manner, which for the practice, particularly in drilling, is of special importance. With such an occurrence the risk in warching for oil is much smaller, and the possible success, after the expenditure of time, labor and capital, can be calculated with greater certainty than with the vein-like occurrence of oil, as may be seen from the accompanying sketch, Fig. 1.

The original deposit A B C is concordant with the dead rock-strata S S, S, accompanying it. From the outcrop of the latter a picture of the trend or bearing of the deposit after it has been found, may be made, for instance, at B, through the shaft DB, since its trend must be parallel to a b c. If then at E a bore-hole be commenced, its depth E F can be quite

accurately determined. This depth is not only a guide for the preliminaries of time and money, but under certain conditions also determines the mode of working, etc. Besides, when the deposit is not found after the bore-hole has reached a certain depth, and a study of the geological conditions shows that there is a disturbance, for instance a fault, between B and F, it is then known that the deposit does not extend thus far, but has previously dwindled away.

With a vein-like occurrence of the oil the technical questions are quite different. At G an oil well corresponding to a cleft has been discovered, and the latter has been exposed. Hence a bore-hole is commenced at K in the hope of reaching the oilcleft (the oil-vein) at 7. The latter, however, at a certain depth turns round, a change which could not be foreseen even by the most thorough geological investigation. The bore-hole misses the cleft, and no result is attained if the primary deposit is at such a depth that it cannot be reached with the drill. If such unfortunate incidents happen in searching for oil, a territory is frequently abandoned as unprofitable, although it may contain considerable quantities of oil.

From the foregoing explanations the importance of correctly deciding the question whether the oil occurs in a primary or secondary deposit will be understood.

In the fissures the oil, if there be sufficient pressure, may rise to the surface, where it can be determined, and, under suitable conditions, extensive accumulations may be formed on the surface near the oil wells, as, for instance, the pitch lake of the island of Trinidad. If, however, the outcrop of these fissures K, Fig. 2, is covered with loose masses of rock (debris, sand, etc.,) the oil spreads out in the latter and forms accumulations like water undrground. In Canada such accumulations are called surface wells. If, however, the oil passes out from the rock-fissure K in a declivity covered with conglomerate D, Fig. 3, it will run down along the bottom of this covering, and, on the foot of it, as an oil-spring Q, such occurrence having to be taken into consideration in searching for oil. These accu