The Economic Aspect of Geology

by C. K. Leith

White has further developed the important principle that, in the geochemical stage of development, both coal and oil react to physical influences in much the same way; and that therefore when both are found in the same geologic series, the degree of concentration of the coal, measured by its percentage of carbon, may be an indication of the stage of development of the oil. More specifically where the coal contains more than 65 to 70 per cent of fixed carbon, chances for finding oil in the vicinity are not good (though commercial gaspools may be found), probably for the reason that the geochemical processes of distillation have gone so far as to volatilize the oils, leaving the solid residues in the rock. White also finds that the lowest rank oils, with considerable asphalt, are found in regions and formations where the coal deposits are the least altered, and the lighter, higher rank oils, on the whole, where the coal has been brought to the correspondingly higher ranks; in other words, up to the point of complete elimination of the oil, improvement in quality of the oil accompanies increased carbonization of the coal. The principle, therefore, becomes useful in exploration in geologic series where oil is associated with coal. Where the coal is in one series and the oil in another, separated by unconformity (indicating different conditions of development), the principle may not hold, even though there is close geographical association.

The oil and gas distillates migrate upward under gas pressure and under pressure of the ground-water. If there are no overlying impervious beds to furnish suitable trapping conditions, or conditions to retard the flow, the oil may be lost. The conditions favorable for trapping are overlying impervious beds bowed into anticlines, or other structural irregularities, due either to secondary deformation or to original deposition, which may arrest the oil in its upward course. A dome-like structure or anticline may be due to stresses which have buckled up the beds, or to unequal settling of sediments varying in character or thickness; thus some of the anticlinal structures of the Mid-Continent field may be due to settling of shaley sediments around less compressible lenses of sandstone which may act as oil reservoirs, or around islands which stood above the seas in which the oil-bearing sediments were deposited and on the shores of which sands capable of acting as oil reservoirs were laid down. Favorable conditions for trapping the oil may be furnished by impervious clay "gouges" along fault planes, or by dikes of igneous rock. Favorable conditions may also be merely differences in porosity of beds in irregular zones, determined by differences either in original deposition or in later cementation.

The thickness of oil-producing strata may vary between 2 or 3 feet and 200 feet. The porosity varies between 5 and 50 per cent. In sandstones the average is from 5 to 15 per cent. In shales and clays, which are commonly the impervious "cap-rocks," porosity may be equally high, but the pores are too small and discontinuous to permit movement.

When the impervious capping is punctured by a drill hole, gas is likely to be first encountered, then oil, and then water, which is usually salty. The gas pressure is often released with almost explosive violence, which has suggested that this is an important cause of the underground pressures. It has been supposed also that the pressures are partly those of artesian flows. The vertical arrangement of oil, gas, and water under the impervious capping is the result of the lighter materials rising to the top. In certain fields, oil and gas have been found in the tops of anticlines in water-saturated rocks, and farther down the flanks of folds or in synclines in unsaturated rocks.

The localization of oil pools is evidently determined partly by original organic deposition, often in alignment with old shore lines, and partly by the structural, textural, and other conditions which trap the oil in its migration from the source.

Effect of differential pressures and folding on oil genesis and migration. Another organic hypothesis proposed somewhat recently[23] is that oil is formed by differential movement or shearing in bituminous shales, which are often in close relationship with the producing sand of an oil field, and that the movement of oil to the adjacent sands is accomplished by capillary pressure of water and not by ordinary free circulation of water under gravity. The capillary forces have been shown to be strong enough to hold the oil in the larger pores against the influence of gravity and circulation. The accumulation of the oil into commercial pools is supposed to take place in local areas where the oil-soaked shale, due to jointing or faulting, is in direct contact with the water of the reservoir rock. This suggests lack of wide migration. This hypothesis is based on experimental work with bituminous shales. The general association of oil pools with anticlinal areas is explained on the assumption that anticlines on the whole are areas of maximum differential movement, resulting in oil distillation, and that they are ordinarily accompanied by tension joints or faults, affording the conditions for oil migration. Data are insufficient, however, to indicate the extent to which the anticlinal areas are really areas of maximum shearing. As regards the exact nature of the process, it is not clear to what extent differential movement may involve increase in temperature which may be the controlling factor in distillation,—although in McCoy's experiment oil was formed when no appreciable amount of heat was generated.

The development of petroleum by pressure alone acting on unaltered shale, as shown by these experiments, has been taken by White[24] to have a significant bearing on the geochemical processes of oil formation. Under differential stresses acting on fine-grained carbonaceous strata under sufficient load, there is considerable molecular rearrangement, as well as actual movement of the rock grains,—thus promoting the distillation of oil and gas from the organic matter in the rocks, and the squeezing out of the oil, gas, and water into adjacent rocks, such as coarse round-grained sandstones and porous limestones, which are more resistant to change of volume under pressure. Migration, concentration and segregation of the oil, gas, and water is supposed to be brought about, partly through the effect of capillary forces—the water, by reason of its greater capillary tension, tending to seize and hold the smaller voids, and thus driving the oil and gas into the larger ones—and partly through the action of gravity.

White also suggests that the process may go further where the parent carbonaceous strata are of such thickness and under such load of overlying rocks that they undergo considerable interior adjustment and volume change before yielding to stress by anticlinal buckling,—than where the strata yield quickly. It is not clear to the writer that the interior adjustment assumed under this hypothesis is necessarily slowed up or stopped by anticlinal buckling. Interior stresses are inherent in any sedimentary formation, when settling and consolidating and recrystallizing under gravity, and these may be independent of regional thrusts from without.

The first oils evolved by pressure from the organic mother substance are probably heavy, the later oils lighter, and the oils from formations and regions where the alteration is approaching the carbonization limit are characteristically of the highest grade. This is the reverse of the order of products obtained by heat distillation. Whether there is also a natural fractionation and improvement of the first heavy oils as they undergo repeated migrations is not known.

Inorganic theory of origin. Another theory of the source of oil has had some supporters, although they are much in the minority. This is the so-called "inorganic" theory, that oil comes from magmas and volcanic exhalations. In support of this theory attention is called to the fact that igneous rocks and the gases associated with them frequently carry carbides or hydrocarbons; that many oil fields have a suggestive geographic relationship with volcanic rocks; and that certain of the oil domes, as for instance in Mexico, are caused by plugs of igneous rocks from below. It has been suggested that deep within the earth carbon is combined with iron in the form of an iron carbide, and that from the iron carbide are generated the hydrocarbons of the oil, either by or without the agency of water. Iron carbide is magnetic, and significance has been attached to the general correspondence between the locations of oil in the western United States and regions of magnetic disturbance.