Limestone. Limestones consist of calcium carbonate (CaCo3) with impurities. The calcium carbonate may be from various sources, quite an amount of it is obtained from marine animals. Limestones are generally deeper water deposits, and therefore carry animal and plant remains or fossils; the study of which enables us to recognize and correlate formations from different parts of the globe. A calcium carbonate rock is too compact to be considered as oil and gas reservoir rock; however, the dolomitic form of limestone composed of calcium magnesium carbonate (CaMgCo3) is porous enough if the magnesium contents are at least 25 per cent. A fossiliferous limestone is not porous enough for oil and gas accumulations, and a pure calcium carbonate lime stone can only be considered as a possible reservoir rock if it is sufficiently water channeled, or if it is fissured.
Coal. Depositions of carbonaceous matter, such as coal and lignite, are formed from vegetable matter and are quite commonly found in a number of the oil fields. They make excellent key-horizons for the driller and geologist. In a few places in West Virginia and Ohio they have been known to con tain oil and gas in considerable quantities.
Glacial Deposits. In countries within the limits of former glaciation, great deposits of sands and gravels, as well as boulders, may be found that have been accumulated from glaciers. The
glaciers may have obliterated former hills or formed valleys and at the time of their melting, the sands and gravels carried by them filled up the valleys and covered the existing outcropping rocks, thus making the finding of such outcrops difficult; there fore glacial deposits are rather a hindrance to the prospector. They do not contain oil and gas, but have in few instances• (Lodi, 0.) served as barriers to movements of oil and gas, pre venting their escape at the outcrop of the oil containing rock.
Regarding shales, sandstones and limestones, it is rather difficult sometimes to draw a distinct line between them, as they may readily grade into one another, forming shaly sand stones, sandy shales, calcareous shales,• or shaly limestones; calcareous sandstones and sandy limestones. Such are known as transitory rocks.
To the uninitiated these great deposits appear to be nothing but a heap of rocks without any semblance of order, yet from this seeming chaos not only have the rocks been systematized but have been arranged in order of their ages or succession of deposition. This was made possible by the study of the fossils found in the rocks.