Natural gas accumulates in reservoirs that are typically found between 3,000 and 25,000 feet below the earth’s surface. Gas may be found in “conventional” or “unconventional” formations as shown in the illustration below.
Conventional natural gas reservoirs are geologic traps in which an impermeable rock traps gas that has collected in pores in permeable rock. Over time the gas migrates up to the impermeable cap through cracks in the rock because the gas is less dense than other materials in the rock. When water is present in the formation, the lighter gas will displace the water to the bottom of the permeable layer. Conventional natural gas is typically found in sandstone beds and carbonate rock. Reservoirs may contain just natural gas, in which case the gas is called non-associated, or may contain both gas and oil, in which case the gas is called associated. Wells are drilled into these reservoirs and natural gas flows upward from the high-pressure condition in the buried reservoir to the lower pressure condition at the wellhead (the top of the well at the surface). Natural gas also accumulates in other types of formations resulting in what is called unconventional gas. Gas in these formations includes:
- Tight sands gas — formed in sandstone or carbonate (called tight gas sands) with low permeability which prevents the gas from flowing naturally.
- Coalbed methane (CBM) — formed in coal deposits and absorbed (meaning attached to solid particles instead of occupying pores in the rock).
- Shale gas — formed in fine-grained shale rock (called gas shales) with low permeability and absorbed by clay particles or held within minute pores and microfractures.
- Methane hydrates — trapped in water with crystalline ice-like substances.
Both conventional and unconventional sources, except for methane hydrates, have become important resources in recent years.