Shale gas is simply natural gas that is found saturated into shale formations. Shale is a type of sedimentary rock, meaning it is formed from layers of compressed sediment. As existing rock formations are exposed to the elements, they weather and break down, forming debris, or sediment, that is deposited across the earth in layers by the movement of wind and the water in streams, rivers, and oceans. As time goes by, more layers accumulate, burying the lower layers deeper and deeper, eventually subjecting them to extreme pressure. Over millions of years, these layers are actually compressed into solid rock. This is how shale is formed.
Underground shale formations are the result of compressed layers of silt, clay, or mud1. Silt, clay, and mud are all made up of extremely fine particles–less than 1/16 of a millimeter in size1. As a result, shale is dense with an extremely low permeability for a sedimentary rock.
Why is natural gas found in shale?
Along with rock sediment, organic sediment also accumulates in layers across the earth’s surface. As plants and certain microbes grow, they store energy harvested from the sun in the form of carbon compounds. When these living organisms die, this organic carbon-based material accumulates and is eventually compressed by new layers, just like rock sediment. Sufficient application of pressure and heat triggers chemical reactions that break down complex organic compounds into natural gas.
With that in mind, two main factors contribute to the prevalence of shale gas. On the earth’s surface, environments that are conducive to the production of shale sediment are also areas that accumulate a lot of organic sediment. In other words, silt, clay, and mud are found in wet areas. Wet areas are usually highly populated with plant and microbial life that will provide the organic material needed for the generation of natural gas. Under the earth’s surface, conditions needed to form natural gas and shale are also similar. If underground pressures are high enough to create a dense sedimentary rock like shale, they are also high enough to initiate the chemical reactions that create natural gas from organic sediment. Shale is considered a natural gas source rock because the conditions for natural gas generation go hand in hand with the conditions for shale production.
How do we produce shale gas?
Traditionally, natural gas has been harvested from easily accessible reservoirs that occur in formations of such materials as sand, sandstone, or dolomites. These conventional reservoirs occur when shale gas migrates into more porous rock types. A well can then be drilled vertically into the reservoir and gas may flow through the rock and up the wellbore.
However, as conventional reservoir supplies were gradually exhausted, oil and gas companies began looking for ways to produce shale gas from its source formation. The development of fracking technology in the 1950s made this a feasible possibility. While shale does not naturally have sufficient permeability to allow natural gas flow within the rock, hydraulic fracturing increases permeability sufficiently to allow fluid to flow up the wellbore. In recent years, horizontal drilling has also seen increasing use in conjunction with fracking to allow increased access to shale gas. Horizontal drilling involves drilling down vertically into a shale layer. The wellbore is then extended horizontally within the layer, allowing increased access to the shale formation, or unconventional reservoir.
Shale gas in the United States
Although the continental US boasts an abundance of shale plays, these resources were largely ignored during the 20th century as industry deemed them uneconomical to produce. However, as the practice of hydraulic fracturing rapidly became more common throughout the 1950s and 1960s, unconventional shale reserves began to gain attention as potentially attractive sites for direct production. In the mid 1970s, the US Department of Energy and the Gas Research Institute formed a partnership with several private drilling companies, with the goal of developing technology that would enable the economical production of gas from the Devonian Shale in the eastern US4. This partnership provided advancements in equipment and technology that made possible increased horizontal drilling.
The prospect of horizontal drilling in combination with fracking raised hope for the potential success of large scale, commercial production of gas from deep shale formations. During the 1980s and 1990s, the Mitchell Energy and Development Corporation became the first company to experiment with commercial shale gas production in the Barnett Shale play in northern Texas4. As Mitchell Energy and Development realized successes in production and profit, oil and gas companies flocked to the Barnett play in such numbers that the Barnett region alone was producing .5 trillion annual cubic feet of natural gas by 20054.
Considering those numbers, it is remarkable that shale gas production in the US has in fact seen even more dramatic gains over the last 5-10 years. Between 2006 and 2010, total US shale gas production rose from 1 trillion annual cubic feet to 4.8 trillion cubic feet per year4. Furthermore, pursuant to this trend, recent discovery of US shale plays have put the national estimate of recoverable shale gas reserves at 862 trillion cubic feet5. The vast majority of these estimated resources are distributed between 7 main shale formations, the largest of those being the recently discovered Marcellus Shale in the eastern US4.
1) Lutgens, F., & Tarbuck, E. (2000). Essentials of geology. (7 ed.). Prentice Hall.
2) Keddy, C. (2009, May 12). Shale gas 101.
3) King, H. (n.d.). Shale.
4) US Energy Information Administration,(2011). Review of emerging resources: Us shale gas and shale oil plays
5) US Energy Information Administration, (2011). World shale gas resources: An initial assessment of 14 regions outside the united states