Tight gas and shale gas
Tight gas is found trapped in impermeable rock and non-porous sandstone or limestone formations, typically at depths greater than 10,000 feet below the surface. The viability of sandstone reservoirs is determined by their porosity, or the open space between grains, and permeability, or how easily fluid or gas moves through the rock. In some cases, the gas can be found in small, isolated zones within 20 feet of each other, but due to the density of the rock formation, are inaccessible via the same vertical well.
The United States has been producing tight gas for more than four decades, and it now accounts for approximately 40 percent of the nation’s unconventional gas output.
Shale is one of the Earth’s most common sedimentary rocks. It is a fine-grain rock composed mainly of clay flakes and tiny fragments of other minerals. Shale can be a gas reservoir, but only formations with certain characteristics are viable for development.
Thermogenic (from the Greek word meaning ‘formed by heat’) gas forms when organic matter in shale is broken down at high temperatures, often a result of burial deep underground. The gas is then reabsorbed by organic material to trap the gas within the shale.
Shale gas is the most commonly known unconventional gas. The United States has experienced a shale gas revolution, in which shale gas production increased from 11 percent of overall U.S. gas production in 2008 to more than 20 percent in 2010, and it may approach 50 percent by 2035.
Globally, initial studies have identified nearly 700 shales in 142 basins around the world.