Mcf: An abbreviation denoting a thousand cubic feet of natural gas. It’s a unit of measurement for natural gas wells; for example, one that produces 600 Mcf of gas per day operates with a daily production rate of 600,000. A single Mcf is equal to approximately 1,000,000 British thermal units (Btu) of energy. The "M" in Mcf refers to one thousand, as in the ancient Roman letter M. One million cubic feet is instead denoted as MMcf.
Source: Investopedia