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Shale |
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Background | Name | Sources | Uses | Substitutes and Alternative Sources |
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Shale is a detrital sedimentary rock composed of very fine clay-sized particles. Detrital sedimentary rocks are sedimentary rocks composed of the weathered and eroded particles of larger pieces of rock. Clay forms from the decomposition of the mineral feldspar. Other minerals present in shale are quartz, mica, pyrite, and organic matter. Shale forms in very deep ocean water, lagoons, lakes and swamps where the water is still enough to allow the extremely fine clay and silt particles to settle to the floor. Geologists estimate that shale represents almost ¾ of the sedimentary rock on the Earth’s crust. Geologists are specific about the definition of the rock called “shale.” Shale is composed of clay-sized particles that are less than 0.004 mm in size. Siltstone is composed of particles that are between 0.004 and 0.063 mm in size. When the sedimentary rock is a mixture of clay and silt, geologists call the rock mudstone. Layers of other sediments eventually cover the silt and mud that collects on ocean and lake floors. The weight of these sediments compacts the mud leading to lithification (lithification literally means turning to stone). The lithification process creates very fine layering in the shale. This layering is called lamination. Shale splits easily into relatively thin sheets due to this lamination. Shale can be red, green or black. The different colors are due to different minerals in the shale. Black shale typically has a very high content of oily kerogen. Kerogen is organic matter trapped in the sediments that is the remains mostly of plants and some water-born microorganisms. Kerogen is not oil, but is thought to be the material that, through complex geological processes, becomes oil. Though still economically unfeasible, a process of heating (in an oxygen-depleted environment) can remove kerogen from shale in the form of liquid oil and natural gas. Shales are very common in the continental crust all over the Earth. In the United States, significant deposits of oil shale are found in the western states. It is estimated that the world’s largest oil field is found in the oil shales under northwestern Colorado. The western U.S. oil shales only cover approximately 17,000 square miles, a relatively small geographical region (including the states of Colorado, Wyoming and Utah). They are very thick, however, and as a result they hold a tremendous reserve of oil, a reserve that represents nearly ¾ of the world’s recoverable oil shale reserves. Shale is too soft and too easily broken into small pieces to be used as dimension stone or even as crushed stone (although some shale is used as “slate” for garden walkways and paving stones). The greatest potential use of shale today is as a new source of oil. It is presently estimated that 1.75 x 1015 barrels of oil are trapped in the world’s oil shales. This is 100 times the total liquid petroleum geologists expect will be removed from known oil reserves. There are many significant problems removing oil from oil shales. Environmental considerations as well as complicated technical problems make it far too expensive and presently unrealistic to remove large quantities of oil from shale. Substitutes and Alternative Sources There are plenty of alternatives to shale for crushed stone applications (for example, road and highway construction and repair) such as limestone, sandstone, quartzite, and granite. These alternatives are so abundant that there is little need to consider or use shale in these applications. As mentioned above, shale’s physical properties do not lend it to be useful as dimension stone: it is simply too soft and its laminations cause it to break into thin layers much too easily. There is little need to consider any kind of substitute or alternative for shale since, presently, it has no important use or application. By Darryl Powell Darryl Powell is a 1984 graduate of the University of Rochester, Rochester, New York with a B.S. in Geological Sciences. For six years Mr. Powell was an Adjunct Professor at Finger Lakes Community College in Canandaigua, New York, where he taught “Introduction to Physical and Historical Geology.” He creates Earth Science educational kits for NeoSCI, Inc., a science education resource company based in Rochester, New York. He also writes Earth Science materials for NewPath Learning, Inc. based in Victor, New York. Mr. Powell is the founder of Diamond Dan Publications, a small company focused on creative activity books and activity-based learning games about rocks and minerals, based in Manchester, New York. |
