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Oil well pumpjack, southwest of Grover, Weld County, Colorado, March 2018. ColoradoPhoto credit: Michael O'Keeffe for the CGS.

Oil and Natural Gas


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Oil forms from kerogen, a mixture of organic compounds in sedimentary rocks. It is most abundant in shales. Shales are analyzed and characterized as potential “source rocks” for oil based largely on their TOC (total organic content).

For kerogen to be transformed into oil, it must be buried to a depth where the temperature and pressure are sufficiently high to convert the kerogen into oil. The place where the depth is sufficient to achieve this, is called the “oil window”. Oil shale which contains only kerogen was never buried deeply enough to generate oil. Therefore, humans must artificially convert the kerogen in oil shale into oil at high temperatures.

Oil shale is different from shale oil. People are increasingly developing shale oils such as the Bakken shale oil in North Dakota and Niobrara shale oil in Colorado. These strata were buried deeply enough to convert the kerogen into oil. However, substantial quantities of the generated oil remained in the source rock, rather than migrating out of the source rock into a more conventional reservoir rock. So, oil companies are successfully using fracking technology and horizontal drilling to extract the oil from these units of shale oil.

Natural gas, in itself, might be considered an uninteresting gas—it is colorless, shapeless, and odorless in its pure form. Quite uninteresting—except that natural gas is combustible, abundant in the United States and when burned it gives off a great deal of energy and few emissions. Unlike other fossil fuels, natural gas is clean burning and emits lower levels of potentially harmful byproducts into the air. We require energy constantly, to heat our homes, cook our food, and generate our electricity. It is this need for energy that has elevated natural gas to such a level of importance in our society, and in our lives.