Types of Natural Gas Resources

The natural gases can be classified as conventional natural gas, gas in
tight sands, gas in tight shales, coal-bed methane, gas in geopressured reservoirs, and gas in gas hydrates.
Conventional natural gas is either associated or nonassociated gas. Associated or dissolved gas is found with crude oil. Dissolved gas is that portion of the gas dissolved in the crude oil and associated gas (sometimes
called gas-cap gas) is free gas in contact with the crude oil. All crude oil
reservoirs contain dissolved gas and may or may not contain associated
gas. Nonassociated gas is found in a reservoir that contains a minimal
quantity of crude oil. Some gases are called gas condensates or simply
condensates. Although they occur as gases in underground reservoirs,
they have a high content of hydrocarbon liquids. On production, they may
yield considerable quantities of hydrocarbon liquids.
Gases in tight sands are found in many areas that contain formations generally having porosities of 0.001 to 1 millidarcy (md). Within the United
States, the largest portion of the gas resource is found in the Green River
Basin of Wyoming, the Piceance Basin of Colorado, and the Unita Basin
of Utah (Ikoku 1984). At higher gas permeabilities, the formations are
generally amenable to conventional fracturing and completion methods.
Gases in tight shales are found in the eastern United States (Kentucky,
Ohio, Virginia, and West Virginia). Of these, eastern Kentucky and
western West Virginia are considered the most important. The shale is
generally fissile, finely laminated, and varicolored but predominantly
black, brown, or greenish-gray. Core analysis has determined that the
shale itself may have up to 12 percent porosity, however, permeability
values are commonly less than 1 md. It is thought, therefore, that the
majority of production is controlled by naturally occurring fractures and
is further influenced by bedding planes and jointing (Ikoku 1984).
Coal-bed methane is the methane gas in minable coal beds with depths
less than 3,000 ft. Although the estimated size of the resource base seems
significant, the recovery of this type of gas may be limited owing to practical constraints.
In a rapidly subsiding basin area, clays often seal underlying formations
and trap their contained fluids. After further subsidence, the pressure and
temperature of the trapped fluids exceed those normally anticipated at reservoir depth. These reservoirs, commonly called geopressured reservoirs,
have been found in many parts of the world during the search for oil and
gas. In the United States they are located predominantly both onshore and
offshore in a band along the Gulf of Mexico (Ikoku 1984). In length, the
band extends from Florida to Texas; in width, it extends from about 100
miles inland to the edge of the continental shelf.
Gas hydrates, discovered in 1810, are snow-like solids in which each
water molecule forms hydrogen bonds with the four nearest water molecules to build a crystalline lattice structure that traps gas molecules in its
cavities (Sloan 1990). Gas hydrates contain about 170 times the natural
gas by volume under standard conditions. Because gas hydrate is a highly
concentrated form of natural gas and extensive deposits of naturally
occurring gas hydrates have been found in various regions of the world,
they are considered as a future, unconventional resource of natural gas.