Shale Gas
Introduction
Shale, which consists mainly of consolidated clay-sized particles, is the Earth’s most common sedimentary rock. Shale looks like the slate of a chalkboard and generally has ultralow permeability. In many oil fields, shale forms the geologic seal that retains the oil and gas within producing reservoirs, preventing hydrocarbons from escaping to the surface. In a handful of basins, however, layers of shale — sometimes hundreds of feet thick and covering millions of acres — are both the source and reservoir for natural gas.

Gas shale reservoirs in the United States tend to be found within three depth ranges between 250 and 8,000 feet (ft.). The New Albany and Antrim shales, for example, have some 9,000 wells in the range of 250 to 2,000 ft. In the Appalachian basin shales and the Devonian and Lewis shales, there are about 20,000 wells from 3,000 to 5,000 ft. Although the Barnett and Woodford shales are much deeper, the Caney and Fayetteville shales are from 2,000 ft. to 6,000 ft, with most of the reservoirs between 2,500 and 4,500 ft. A good shale gas prospect has a shale thickness between 300 and 600 ft.

Shale has such low permeability that it releases gas very slowly, which is why shale is the last major source of natural gas to be developed. The good news is that shale can hold an enormous amount of natural gas. The most prolific shales are relatively flat, thick and predictable, and the formations are so large that their wells will continue producing gas at a steady rate for decades. (Sclumberger, October 2005, “When Your Gas Reservoir is Unconventional, So is Our Solution.”)

  • Shale is the most common sedimentary rock in the world.
  • Today, shale gas sources are making a huge impact in North American natural gas supply.
  • A new groundbreaking study from Navigant Consulting Inc, commissioned by the American Clean Skies Foundation estimates more than 2,247 Tcf, or 118 years at current production levels of natural gas from shales.
  • Unconventional gas, especially shale, has ramped up sharply over the past several years, in terms of annual production and in terms of economically recoverable reserves.
  • Rapid escalation of unconventional production observed historically is continuing, and the unconventional resource base appears adequate to support that escalation to allow significantly increased volumes of unconventional production to continue for decades.
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