Unconventional gas resources con-tinue to be a growing part of the total gas production in the United States and have captured the interest of energy firms around the world. Unconventional resources comprise three major categories; tight-gas sand-stones (low-permeability), shale gas, and coal-bed methane. Tight-gas reservoirs currently comprise approx-imately 35% of the uncon-ventional gas production in the U.S. Lower 48 (Wood Mackenzie's Unconventional Gas Service).

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