natural gas
(3) (A) Except as provided in subparagraph (B), the term “natural gas” means any fuel consisting in whole or in part of— (i) natural gas; (ii) liquid petroleum gas; (iii) synthetic gas derived from petroleum or natural gas liquids; or (iv) any mixture of natural gas and synthetic gas. (B) The term “natural gas” does not include— (i) natural gas which is commercially unmarketable (either by reason of quality or quantity), as determined under rules prescribed by the Secretary; (ii) natural gas produced by the user from a well the maximum efficient production rate of which is less than 250 million Btu’s per day; (iii) natural gas to the extent the exclusion of such gas is provided for in subsection (b); or (iv) synthetic gas, derived from coal or other alternate fuel, the heat content of which is less than 600 Btu’s per cubic foot at 14.73 pounds per square inch (absolute) and 60 degrees Fahrenheit.