GEOLOGICAL POSITIONS OF ROCK-OIL.
Figure 189 illustrates the several horizons of petroleum and the hydro-carbons gene rally. We have made the gas and oil reservoirs or strata much larger in proportion than they exist in nature; but we wish to convey the idea rather than the actual status,—which, we must state, can only be conjectural. A stratum of oil, even if large enough to supply for years our great flowing wells, would appear as a faint line only if drawn to a scale on our section; and the thin leaders or jets of gas ascending through the earth, if made in proportion to their actual sizes, would scarcely be seen. In this figure, a represents the general level of the Great Kanawha near Charleston; b, the region of heavy petroleum; c, the general level of Oil Creek; d, the region of the second oil; and e, the lower oil, or gas.
These horizons of oil seem to be general, but they are not invariably of the consist ence here specified. For instance, the lower oil formation existing in the Corniferous limestone of the upper Silurian rocks may be 2000 feet deep on Oil Creek and 2700 feet beneath the Great Kanawha, and its hydro-carbon exist only as gas. But in cer tain portions of Kentucky this rock appears to come near the surface, and in Canada it crops out. In the oil-regions of Chatham, in the vicinity of Lake St. Clair, Canada, it is very productive. The wells there are from 300 to 500 feet deep, and the oil pro duced is a fair petroleum, of about the same density as that produced in Oil Creek, Pennsylvania, at the same depth. The second oils, in the shallow wells of Northwestern
Pennsylvania, are only from 33° to 38° Beaume's hydrometer, and in the deep wells from 46° to 50°; but under the Great Kanawha it exists principally as gas or the lightest naphtha. It thus appears that the density of the oil depends more on its proximity to the surface than on the geological horizon in which it exists.
As before observed, the volatile parts escape when exposed, or when the means of escape are offered, and condensation takes place in consequence. On long exposure, nothing but the solids is left, and whether exposed to a moderate heat or cold, the same thing happens; but under a low temperature bitumen is left, composed of carbon, hydrogen, and oxygen, correspond ing to the bituminous coals of the West; under a moderately high temperature, a comparatively pure carbon is left, corresponding to the anthracite coals of Pennsylvania; but if the heat be excessive, even the carbon becomes or remains vola tile, and no residue remains. This is demonstrated by the fact that heat will change the hardest anthracite to vapor, — a chemical operation which we may see effected every day by the combustion of coal.