2. A reservoir above, in which they have gathered. This is either a porous sandstone or a porous or creviced limestone.
3. Oil and gas are lighter than water, and are usually under pressure owing to artesian water. Hence, in order to hold them from escaping to the surface, the reservoir must have the shape of an anticline, dome, or lens.
4. It must also have an impervious cover, usually a shale. In these reservoirs gas is under a pressure which is often enormous, reaching in extreme cases as high as a thousand five hundred pounds to the square inch. When tapped it rushes out with a deafening roar, sometimes flinging the heavy drill high in air. In accounting for this pressure we must remember that the gas has been compressed within the pores of the reservoir rock by artesian water, and in some cases also by its own expansive force. It is not uncommon for artesian water to rise in wells after the exhaustion of gas and oil.
Life of the Ordovician
During the ages of the Ordovician, life made great advances. Types already present branched widely into new genera and species, and new and higher types appeared.
Sponges continued from the Cambrian. Graptolites now reached their climax.
Fig. 278. Stromatopora
Stromatopora—colonies of minute hydrozoans allied to corals—grew in places on the sea floor, secreting stony masses composed of thin, close, concentric layers, connected by vertical rods. The Stromatopora are among the chief limestone builders of the Silurian and Devonian periods.