Unconventional gas, also referred to by geologist as "continuous gas" or "blanket gas", is methane or another light hydrocarbon similar to that found in the conventional anticlinal trap, but it is stored in the earth and produced differently. It is stored uniformly in a formation that extends over a wide area, but is trapped in a rock formation that requires additional resources to free it.  In the broadest sense, unconventional natural gas is gas that is more difficult to extract. Essentially, there are six main categories of unconventional natural gas. These are:

• deep gas
• tight gas
• gas containing shales
• coalbed methane
• geopressurized zones
• Arctic and sub-sea hydrates 

Unconventional natural gas constitutes a large proportion of the natural gas that is left to be extracted in North America, and is playing an ever-increasing role in supplementing the nation's natural gas supply. The Department of Energy reports the share of unconventional gas doubled from 17 percent of Lower 48 natural gas supplies in 1990 to 35 percent in 2003. By 2025 it is projected to be 44 percent—matching the role of conventional gas—with the remaining 12 percent of domestic supplies imported.