GENERAL GEOLOGY It is known that the earth is a large sphere of about 8000 miles in diameter, having a comparatively thin rocky crust, or lithosphere, with which the geologist is mainly con cerned, as it is the only accessible portion of the earth for a gen eral study. Oil and gas, as well as our precious metals and min erals, are found in the lithosphere.
The rocks of the lithosphere are divided, according to their mode of origin, into three great classes known as the igneous, metamorphic and sedimentary rocks.
Igneous Rocks. Igneous rocks have been formed by the cooling and solidifying of hot molten masses, becoming crystal line or glassy and compact. Originally they were at very high temperature, so no living animals or plants were in them at the time of their formation, and are, therefore, unfossiliferous, and as they arc found in almost any form and shape imaginable, are spoken of as unstratified rocks. Rocks of this class have a very low percentage of porosity, and are, therefore, unsuitable for oil and gas accumulations. Granite, diorite and basalt are typical rocks of this class.
Metamorphic Rocks. Rocks, whether igneous or sedi mentary, that have undergone some sort of a change or recon struction, either physically or chemically, are known as meta morphic rocks. Metamorphism may be due to various causes, such as the rearrangements of the molecules caused by pressure or by the contact of igneous and sedimentary rocks. Hydration
and dehydration may bring about certain changes; oxidation or reduction as well as percolating waters and gases also cause metamorphism. These rocks are quite compact and crystalline, thus unsuitable for oil and gas reservoirs. It is possible,"however, that dissolving agents, such as percolating waters and gases, acting upon metamorphic rocks, may make them more porous; and such may accumulate oil and gas if in contact with sedi mentary rocks; however, rocks of this class are not important in this connection and may be dismissed as a factor for oil or gas accumulations. Gneiss and schist are examples of metamorphic rocks.
Sedimentary Rocks. Rocks which have been formed from the eroded particles of existing rocks and deposited in layers, generally by water, sometimes by winds or glaciers, are known as sedimentary rocks. (Fig. 4.) They are deposited in nearly horizontal layers and contain the remains of plants and animals, known as fossils, which may be microscopic in size or larger. Sedimentary rocks are porous to a greater extent than are igneous and metamorphic rocks, hence suitable for oil and gas reservoirs.