ACCUMULATIONS OF OIL AND GAS a commercial accumulation of oil and gas may be possible several requirements must be fulfilled. We must have a porous rock such as a sandstone, either the consolidated or unconsolidated type, or a suitable form of limestone to act as a reservoir. These rocks by virtue of their porosity may contain and accumulate oil and gas provided they arc sealed in so that their contents may not be lost by migration or evaporation. Shales play a very important part in this connection, as they act as an impervious seal, and when such a shale body entirely envelops the entire porous reservoir, then accumulations may become possible. A lenticular sandbody is a familiar example of this type, and serves as the reservoir, while the shales form a seal. (Figs. Ioa and rx.) As the porosity of a rock is variable, so that portions of it may be very porous and other parts less so, naturally the most porous portion will be the most suitable for oil and gas accumu lations, while the non-porous, or less porous portion will act as a.
seal. A condition of this kind is due to the differential cementa tion of the sands, and the oil man speaks of the non-porous sand as a tight sand, while the porous one is called an open sand.
Accumulations that are controlled by the porous portion of a sand body are similar in shape to accumulations found in lenticu lar sands, and are also known as lenticular pools, but in such a case the lenticular form refers to the shape of the "Pay" portions of the formation as the entire sand body itself may not be lenticular. From this we may infer that if the central part of a formation has an effective porosity sufficient for commercial accumulations, the surrounding less porous part may act as a seal.
The term "sand" is employed by the practical oil man to indicate the producing horizon regardless of its actual texture; thus it may be a sandstone, limestone or shale, but if producing oil or gas it is called a sand. In this connection sand will be used to indicate a sandstone formation only.
Quite often the rocks in which the accumulations are expected, come to the surface (crop out); the examination of such out crops will generally show signs of petroleum, but as the out cropping permits the escape of the oil from the rock, such a rock will therefore be less important at the point of outcrop than it will be some distance under cover. The escaping oil, however, may form a seal upon evaporation at the outcrop, and permit accumulations a short distance away. There are many instances where oil pools are found close to the outcrop of the sand, but such pools are generally short lived. However, this disadvantage is overcome by the fact that the pools are shallow and may be operated at a comparatively small expense.
Considering the case of a reservoir that is uniformly porous and where oil and gas may migrate great distances, the presence of a fault or intrusive may act as a seal. A fault may bring a comparatively non-porous rock into juxtaposition with the sand or the faulting action may form a gouge which acts as a cement and fill up the affected portion of the sand body. Often a fault
or an extrusive permits the escape of oil and gas from the sand to the surface. (Fig. 1 2 ) Accumulations of a large amount of oil and gas have been made possible by the presence of a suitable geologiCal structure; in other words, where the strata have been folded, and the sand is filled with gas, oil and water, the folding permits the separation of the gas, oil and water according to their specific gravities, and if the sand is porous throughout, the accumulations of gas will be found at the highest point of the structure, the oil above the water level, the water in the lower parts. In the absence of water, the oil may be expected in the lower portion of the structure. These folds or geological structures may be various as anticlines, homoclines, synclines or some variation of them. As these structures are often evident from the surface they are of great importance in locating promising oil and gas territories. (Figs. lob and ri.) The porosity of the rocks is one of the main controlling factors. The larger the pore space the greater the possibility of the movement that may take place in it. The porosity of a rock specimen is determined by the amount of water it will absorb as shown by the weight of the rock before and after saturation. The figure so obtained will represent the theoretical porosity of the specimen, which may vary from 5 to 4o per cent. The most porous sands are those that are made up of grains of uni form size, thus a sandstone whose grains are of different sizes, the smaller grains may fill the spaces between the larger ones, naturally reducing the size of the pore,space. The question of porosity is discussed by R. H. Johnson* as follows: "Pores are of two kinds: Those which are entirely inclosed, so that they do not belong to a system of communicating pores tapped by the bore hole or shot hole; second, those which do so com municate. Porosity, of course, includes both. But since pores of the first class are valueless to us, we should confine our atten tion to the latter, for which I propose the term 'effective poros ity.' " Various calculations have shown that the maximum effective porosity of an oil sand of the consolidated type is 6 per cent of the whole, and in many fields it is much less. Only a portion of the actual amount of oil in a sand is obtained with the present producing methods. All the oil is not recovered because the movement of oil from the pores of the sand into the bore hole is caused by the pressure of the gas with the oil, and when this pressure has declined the flow of oil is stopped and the wells production becomes so small as to be no further profitable. So it will be seen that although a well may be abandoned as no longer a commercial venture, still there may be a great amount of oil in the sand that has not been recovered. The question of porosity is becoming more and more important as the known fields are depleted, and further recovery of oil is made possible by flooding the sand or by artificial restoration of the lost "rock pressure," in order that recovery may be increased.