The giant McElmo Dome Field (CO2) has increased production. As best I can tell it still has at least 40 years remaining reserves. CO2 is also being sought by many companies to use in tertiary recovery. (Enhanced Oil Recovery)
There may be other parts of the county with CO2...and another mineral to really watch for is helium. A critical shortage is being felt and 2% Helium in gas would be well worth scrubbing and selling.