TIGHT GAS. Tight gas is unconventional natural gas which is difficult to access because of the nature of the rock and sand surrounding the deposit. Because this gas is so much more difficult to extract than natural gas from other sources, hydraulic fracturing and directional drilling is necessary to produce the well. This is also 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 GAS:

Shale gas refers to natural gas that is trapped within shale formations. Shales are fine-grained sedimentary rocks that can be rich sources of petroleum and natural gas.  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.

Over the past decade, the combination of horizontal drilling

 and hydraulic fracturing has allowed access to large volumes of shale gas that were previously uneconomical to produce. The production of natural gas from shale formations has rejuvenated the natural gas industry in the United States.

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.