I. noun
: gas issuing from the earth's crust through natural openings or bored wells ; especially : any of various combustible gaseous mixtures that when in the dry state contain largely methane and in the wet state in association with petroleum contain also higher hydrocarbons (as ethane, propane, butanes, and pentanes) and that are used chiefly as fuels directly or by recovery of gasoline or conversion to other liquid fuels and as raw materials for the manufacture of carbon black and many other products (as nitroparaffins and synthesis gas) — see casinghead gas
II. noun
: gas manufactured from organic matter (as coal)